This is nonsense. The desktop environment is a secondary concern when it comes to building a console-like experience, and it being 100% proprietary does not mean it’s not polished.
Don’t let this guy discourage you from trying it. I’ve been using Linux for over 20 years, and the Steam Deck is the most polished experience I’ve seen for gaming on Linux, even for non-Steam games. If you decide it’s too limiting, you can just switch to something else.
Actually, you’re right, I apologize. I had it in my head that it was for a console. For general PC gaming at a desk I agree with Nobara.