So can this non-mindless int -5 creature figure that out? Not clear.
I don't know why they wouldn't: we're talking about the same difference as there is between 13 and 14. Even plants without an Int score are capable of holding a conversation; You can Speak with Plants and that "spell doesn't make them more friendly or intelligent than normal". So -5 is higher than a scoreless bush and that bush could understand basic situations about its environment. The concept of 'up' and using an ability they have are things a plant can do so why would a cave worm struggle?
Also look at Awakened Tree [int -5] "Invested with a workable set of instincts and the ability to mobilize and attack, these awakened trees follow the commands of their master and fight to protect their home." Having Mindless seems to be where they have programmed or limited understanding kicks in.
You have a bit of a point, that typical animal level intelligence is like a -4 int modifier, and that most things with a -5 int modifier are considered mindless (which is what the cave worm has, though it is not mindless).
It's the other way around: for Mindless "Most, if not all, of their mental attribute modifiers are –5." And they don't even follow that rule as I don't recall something with all -5's [or even close*]. There are plenty of creatures with -5 and not Mindless just in Animals.
For instance, Cave Fisher has -5 Int and can lurk, lay in wait for prey and leave decoys. Flash Beetle are listed as being trained and domesticated. Giant Scorpion are trained as pack animals and war beasts. Giant Stag Beetles are uses as mounts, pack animals and war beasts. -5 int creatures seem to be treated any different from -4 for the ability to train, learn and/or understand. It should also be noted that it says "gifted, patient, and brave animal trainers can condition cave worms to serve in all manner of roles, such as living siege engines, shocking methods of executing foes, or merely pets." Even the "notoriously dim witted" Cave Worm isn't Mindless.
* While Mindless says mental stats are usually -5, only Int is usually that. Even your basic -1 Skeleton Guard has -0 in it's other mental stats.
I don't disagree with you necessarily.
But that all boils down to me saying not clear.
Wis governs instincts based on what I can see. And there is only one creature printed that has a wis below -2. Living Sap, that thing is the int -5 and wis -5 example, it just exists and anything ending up in its path it consumes.