He means they have a problem with Linux users. What other reason would there be to buy up games and remove native Linux support the second its removed from the steam store? (Rocket League for example)

I think he means the whole “Not enough users to justify porting applications, users don’t use it because applications don’t support it” thing.

The problem is that logic has been dead for years. Users are here. The Steam Deck is wildly popular. Tim Sweeney is just a dumbass.