I'd rule that Mindsight only lets you see creatures you could telepathically connect with, but we're not talking common-sense houserules, so that would probably work.
Biologist here! Lots of animals have digits that look a bit like opposable thumbs, and some actually are opposable. Look at bird talons, for instance. However, most of these would not be capable of using tools or weapons; they could hold a hammer, but they couldn't use it against nails (or skeletons).
Not to mention that the right foreclaw looks an awful lot like the claws of many animals with absolutely no ability to hold things. We have to look at all provided evidence, not just cherry-pick the ones that fit our preferred conclusion.