My understanding is they were never closed source. Some of their code is GPL and some is proprietary, but all is source-available on GitHub. There was a bug where you couldn't build their client without a proprietary dependency, but they have fixed that so you can now build their client with only GPL code again.
To be honest, it looks like he just had an internal model of “internal code no gpl”, “external code gpl” and mindlessly answered based on that. The fact that it made the latter impossible seems to have been successfully impressed on him.
Overall, I’ll stay a Bitwarden customer. People fuck up and I’m a tit-for-tat-with-random-forgiveness tactic user, not grim-trigger.
I could accept that he doesn't understand how open source licenses work, or doesn't care, and that it was not meant as a shady move. But still I wouldn't call it a bug, and it does not inspire confidence. Still it's not LastPass-bad.
This said, I still recommend Bitwarden to my family. I moved to pass (https://www.passwordstore.org/) a while ago just because it corresponds better to my needs and I have more control.
I suppose the reminder here is that seeing does not warrant believing.