None of that makes Bitwarden not open source.
Yes, it does, because it violates its own license GPLv3 by having proprietary build-/runtime dependencies.
If it was under a different, maybe more permissive, open source license, then maybe it would still be open source, but as of right now i likely breaks its own license terms.
Not only that, they specifically state this is a bug which will be addressed.
From what they state, they think that because executables that share internal information via standard protocols does somehow not break GPL3 terms compared to two libraries that share internal state via the standardized C ABI which does. And they seem to not consider that a bug, just the build-time dependency.