Mint user here. I think that's because for gaming, Mint is a great compromise. Debian's ultimate focus is stability, which makes it a fantastic choice for servers, but in gaming, you often want components that aren't quite that old. It still doesn't randomly break your stuff, unlike rolling release distros

That's why I said Debian testing / unstable, not Debian stable. Such kind of approach (whether in Mint or Debian stable itself) can cause problems too unless people understand its limitations.

I periodically see a bunch of people complaining that their hardware doesn't work, which ends up being them using Mint which doesn't ship recent kernel and Mesa.

Rolling flavors of Debian are a better fit in my opinion.

Also, I think KDE is a better fit for modern gaming features, due to Cinnamon being way slower in supporting Wayland. Having focus on its own DE and not keeping up with the times is a downside for Mint. Even Ubuntu stopped its own DE efforts for that reason.

irc every more or less official Debian related place tells you not to use those unless you really know what you're doing

You should know what you are doing no matter what you are using. That's my experience. I'd say Debian testing/unstable isn't any worse than a bunch of other rolling distros, like Arch or what not. If anything, it's more stable than Arch. Those who say not to use it are doing a disservice.