I used to have the option to switch accounts in the YT app itself. But for some reason now YouTube prompts the Google TV account selector instead, and it doesn't show any other secondary accounts already logged on the device (maybe because I don't have the new Multiple Profile support yet?).
UPDATE: I can switch now, seems like my CCwGTV got Multiple Profiles and it picks up other accounts. But my Realme TV Stick still doesn't have it. I hope these are signs that it will roll out to everyone.
I just got this tonight and it's absolutely infuriating. It's impossible to sign in to any other accounts.