Super late to this, but I have a weakness for grammatical puzzles. I think you put your finger on something rather obscure—the sort of thing the people on Language Log would come up with competing explanations for. Here's a guess about what is going on.

The "my" in "my cofounder" is different from how "my" usually works. Normally it expresses an "of" bond, as "my father" means "father of me". But here, it expresses a "with" bond, as in "cofounder with me", not "cofounder of me". Think of "my" as being bound to "co" rather than "founder".

The reason why the "founder of me" meaning doesn't arise here is partly because (as graeme pointed out) we never speak of founding a person in English. But it's also because the phrase already mentions what Scott is a founder of (Skysheet), so "cofounder" has a natural object nearby to bind with.

Compare this to a phrase like "my coworker". There, the "with" meaning is unambiguous, but that's because "work" doesn't normally take an object, so there's no competition for what "my" should bind to.