There’s a good chance there will be a virtuous cycle, where the Steam Deck’s popularity makes it easier to game on Linux for regular PC users too, which will help out everyone gaming on Linux. Especially as Microsoft keeps dicking around with Windows and trying to turn it into a subscription OS and people just get sick of it.
Yeah, that’s where that 5% is coming from. Without that virtuous cycle, I’m guessing it would stay between 2-3%. The Steam Deck is cool, but it’s still fairly niche, and many Steam Deck users don’t intend to ever use Linux on their home PCs (e.g. my coworkers that have them).