You can still add non-Steam programs to Steam to use them that way, so this doesn't even prevent that. This just means Valve is not distributing this over their store.
The Steam Deck community already has scripts for installing & adding to Steam emulators for basically everything in one shot, and Steam isn't doing the distribution of any of these.
Parents comment was only asking why would someone launch dolphin via steam and commented on it being bloat. We weren't talking about distribution because as you pointed out, it has little to do with launching dolphin as a non steam app.
Some setups can be elaborate with Roms listed in Steam / Big Picture with box art, fan art, and various metadata. Software like Steam Rom Manager assist with this.
Considering console emulation is often played with controllers perhaps infront of TV, Steams launcher is probably more suitable for launching emulation than using a keyboard or mouse for a start menu.