Unfortunately the Android Chrome flags in older posts are all gone in current versions of Chrome. Instead, it seems the “Autofill” settings in Android Chrome have to be set to “Autofill using another service”, in addition to the Android system settings set to Bitwarden.
Now passkeys finally work on Android, but, all autofill in Chrome goes through Bitwarden now. It does not seem as robust as Chrome’s own autofill, so still deciding if want Bitwarden passkeys or Chrome autofill, sadly can’t have both at the moment.