They said this is on purpose. You shouldn’t need to autofill credentials from a website not already included in the login. If there were an Autofill button for all websites, it would be easy for someone to not notice they’re on, for example, a phishing website that looks like the real website.
If, on the other hand, the login works on a second website too, then the user can click the same three-dot menu to add the URI to the login item and be sure that whenever Bitwarden suggests them o autofill, it’s almost always safe.