

FOSS is a standardized term. As the Free Software Foundation defines it:
Free and open-source software (FOSS) is software available under a license that gives users the right to use, share, modify, and distribute the software – modified or not – to everyone and provides the means to exercise those rights using the software’s source code.
You are not granted right to modify or distribute Bitwarden. You can inspect and use that to build your own. That is what Vaultwarden does.

All I say is that it’s not FOSS in the strict sense.