To add onto this, if you care about supply chain attacks, bitwarden mobile supports Fdroid builds (albeit not part of the main repo because they rely on xamarin) so you can host your own fdroid repo and run your own builds if so desired.
I think they meant if they don't like the direction that the Android client takes, i.e. they stop allowing you to change the backend url for example in which case, yes you would need to fork or rewrite it