You can access it from the android Bitwarden app. Go to the password generator. Where it says “What would you like to generate?”, tap Password and select username in the popup, then click OK. After that, it gives you an option to use a different emails when it generates the email address to use for the account you’re setting up.
It’s probably because the two instances you named have admins who actively try to moderate problems as opposed to joining in as with lemmygrad. But that’s just my take from it all.