Yes, but they may not be singling out Bitwarden as not being open source. It’s likely just that they use Vaultwarden as it’s more lightweight. Also Vaultwarden is only self-hosted, so you can be sure what code is running on the server, whereas Bitwarden has a hosted option. I’d imagine there’s a way to tell but from my understanding, you just have to trust that they run the code they say they are running.