As for Teams, it looks like it’s much closer to Markdown (uses the same idiosyncratic/stupid link syntax), but still significantly incompatible even if they call it that. And my guess (as a non-user) is that it’s just an input method immediately converted to HTML or similar, not retained as text. So in that way it’s not Markdown either.
I get bold and italic confused because Google Chat is almost-Markdown except for * being bold and _ being italic (whereas it's double vs singular in classic Markdown).
My company is on Teams and I regularly use Markdown in my messages, though I still struggle to remember that I have to use underscores not asterisks for italics.
It is funny to occasionally see it explained like 'on Reddit you can use ...' and think '..dude, markdown, just tell them you can use markdown' (and then realise oh right yeah ok, your way is probably clearer to them and you probably don't know it as 'markdown' either).
Reddit's Markdown flavor is a bit weird though. It got closer to CommonMark with New Reddit, but the rest of the UI got worse, and people using Old Reddit don't get the formatting the new version supports, so things like code blocks are often broken.