Hello, I'm the player that the OP mentions, I'm gonna give my two bits with how I'm interpreting things rules wise, and I am aware my reading of these things is dense so hear me out:
The crux of what I feel is the confusion here is the sidebar, maybe I am misinterpreting it here, but this sidebar in question that the feat mentions is addressing two things:
First paragraph is how concealment is meant to be adapted to non-vision special senses, since things like smoke bombs wouldn't work for echolocation, but a noisy chamber would be the equivalent for it, ie. specifically environmental sources of concealment.
And it specifically tries to pin this down, so that other non-stealth action based foils that can be used against the special sense creature (hence something like a silence spell is compared to invisibility), that ofc COULD help later on should you choose to stealth.
The second paragraph (Using Stealth With Other Senses), which I believe is the main course of what it means to foil said senses is also the main point of contention here, because yes it does mention the intended design for stealth is for visual detection, but right after it also states that for many special senses a player can describe how they circumvent it, in that part nothing states vision based special senses cannot be foiled as well (not to be confused with the first paragraphs talk about non-visual senses, which is specifically talking about adapting preexisting factors as the equivalent obstructions that vision has).
The reading here would imply that something like darkvision, which is both visual and a special sense, has to deal with non-darkness based visual foils, along it's dark sight being potentially foiled.
The short of it about sidebars paragraphs are:
-First one guides the GM to adapt environmental concealment (which is typically visual based) for non-visual senses.
-Second denotes optional rules for players to use their Stealth in attempts to avoid special senses.
What I consider important is how special senses are defined in the main senses section, which includes things like darkvision.
"Special senses allow a creature to ignore or reduce the effects of the undetected, hidden, or concealed conditions when it comes to situations that foil average vision."
From this segment a few things as how I'm interpreting it, and I will grant you are a bit confusing since it expects the reader to indirectly connect the dots here, is for example using cover to get the hidden conditions would be ignored for some special senses as its a average visual foil. Which begs the question, if stealth requires gaining the conditions of cover or concealment to hide or sneak, and should be considered a prerequisite to foiling any type of precise special sense, it means some precise special senses are un-hideable from, since it's possible those preconditions are nigh impossible to fulfill.
(The sidebar example of stepping lightly to avoid being detected by tremorsense could never work since you would need cover or concealment before the fact, which stepping lightly by itself doesn't give, granted it might work for imprecise tremorsense, but not for precise tremorsense)
One take is that, it's implied they are first already benefiting from some form non-vision concealment when doing the "slowing the heartrate" example mentioned in the side bar, but that's never truly elaborated on in there,
OR, the other take is that the act of slowing the heartbeat is meant to first foil the sense (ie. stopping the special senses ability to ignore average visual foils) and then check if the character in question fulfills the typical stealth conditions post foil, which I'd wager typically foiling the sense would grant.
So for the example of darkvision, which normally ignores the conditions of seeing a creature in darkness, would be foiled, granting the creature in question the same concealment a normal creature with average vision would, which fulfills the conditions of stealth.
And I am interpreting this the same for all the other special senses, Ie. a creature with precise tremor sense can have its special sense foiled, and since it might lack other senses, the creature in question would be hidden or concealed for the purpose of stealth.
It doesn't mean of course that it cannot seek with said foiled sense after the fact, I don't think foiling a sense is meant to be read in such a way.
The other rules interpretation from the first take is that, non-vision based precise senses cannot be foiled without outside interference (as mentioned in the first part of the sidebar) or using the create a diversion action first, before trying to sneak to foil them. But what happens after you sneak while foiling? Wouldn't you constantly have to regain concealment if foiling by itself is not meant to give concealment? If so, foiling a precise special sense is useless if you do try to use the create a diversion action, as it would instantly fail after you sneak. This again is also not addressed in the sidebar.
Mind you that would also align with the consensus that darkvision also cannot be foiled properly, specifically because precise special senses , which dark vision is also cannot be foiled.
The only rules way I am weaseling that foiling a special sense is intended to mean creating concealment is that, the term "foil" was also used within the special sense description for average vision, where in it's attempting to connect average visual foils to stealth's predefined stealthing options of covers and concealment. So it would at very least be consistent in terminology use.
And mind you this interpretation and talk isn't merely from my perspective as a player, but also as a potential future GM, as I want to understand how to play it as close to the rules as possible, so i know how to also properly adjudicate it.
Ok, this was my confusing word salad of a take. I realize there is a lot that's basically trying stretch the interpretation of things. But it's also likely the consensus might just be that foiling darkvision and other precise special senses to be super difficult (if not downright unintuitive), which is just as a valid take to have.
