It's Daniel's code (well, I guess and all those contributors, too) and thus can license as they see fit, but the whole idea was to have an alternative implementation. If Bitwarden had heartburn about a lighter-weight GPLv3 implementation existing, they're in for some bad news because there are currently 1200 forks still on the GPLv3 license

I doubt very very seriously that there is overlap between the audience who runs Vaultwarden and those who want to start up a company that sells Vaultwarden as its backend to compete with Bitwarden