just read the Assume Earth's Mantle 14th lvl earth kineticist feat -
If your Strength is below +4, this armor raises your Strength to +4. If your Strength is +4 or higher, this armor grants you a +1 item bonus to your Strength.
what exactly does this do?! does item bonus to an attribute stack with item bonus to checks using that attribute, in this case, for example athletics?
so the athletics modifier would in that case be {str attribute (+ item bonus to str)}+prof bonus + item bonus to athletics (other bonuses/penalties left out for brevity)this would mean you've essentially got 2 item bonuses to that check, which is specifically dissallowed by the CRB rules
There are three other types of bonus that frequently appear: circumstance bonuses, item bonuses, and status bonuses. If you have different types of bonus that would apply to the same roll, you’ll add them all. But if you have multiple bonuses of the same type, you can use only the highest bonus on a given roll—in other words, they don’t “stack.” For instance, if you have both a proficiency bonus and an item bonus, you add both to your d20 result, but if you have two item bonuses that could apply to the same check, you add only the higher of the two.
This appears to be the only item bonus to an attribute we've yet had, is it a mistake? is it intended to essentially be able to get +7 str at lvl 20 if you also have an apex item (even though that was only previously possible if you had your apex item with your key ability modifer)?
P.S. the bold in the quotes is my emphasis
Certainly nothing prevents you from using Assume Earth's Mantle *and* wearing a Major Gate Attenuator. The thing that prevents you from using two Apex items is the Apex trait itself, which Assume Earth's Mantle doesn't have.
The point of the ability is to let the earth kineticist keep pace with classes that have Str as a KAS for things like athletics checks. So it absolutely should not conflict with an Item bonus to athletics (from say, Armbands of Athleticism.)
The game probably assumes you'd rather use the Con boosting item that's for your class, but I don't why you couldn't hit +7 str if you took the Strength Apex item instead. This is basically what Str-classes get when they get a Str apex item, so it's not like this breaks the math.
RAW the one is an Item Bonus and apex items themselves are untyped, so it would Stack
I don't remember how the Remaster wording was though
From the Preview the language in the remaster apex items is, for example, "When you invest the bracers, you either increase your Strength modifier by 1 or increase it to +4, whichever would give you a higher score."
So that should stack with Assume Earth's Mantle.
I'm not sure there's a huge reason to do this though, since you still use Con to hit, for your DCs, etc.