Oh!!!
Very important tidbit about how the Game Share feature works on Switch 2 from the Ask The Developer interview…
https://www.nintendo.com/us/whatsnew/ask-the-developer-vol-16-nintendo-switch-2-part-3/
Game Share uses the same video streaming tech as what the Wii U Gamepad used! Rather than copy the actual game over, the host Switch 2 broadcasts a video stream over WiFi. So that’s where the rumour came from!
That also means that it should indeed be possible for Switch 2 games to use another Switch 1 or 2 as a second screen, Wii U Gamepad style!
Edit: From the same interview…
51 Worldwide Classics does indeed work like a Wii U game on Switch 2! Broadcasting multiple viewpoints to different players!! Second Screen gaming is back!!