In main category leaderboard, everyone's platform is Linux, but I think at least current WR holder is playing on Windows. So why are there so many Linux runs?

We're playing on the Linux version, not the Linux operating system.
So many people seem to misunderstand this tbh
We're running the linux version on Windows.

Linux is faster because on linux and mac versions mad dummy skip is possible, and also has better input handling

basically what the other two said. it's considered the best version to speedrun undertale on, it has better input handling and the glitches you can do, ie Mad Dummy Skip, are either exclusive or preform better on linux.

So, if I use Linux version, can I play Glitchless?

Yeah