What features will depend on the close-source part, and which do not?
There are definitely some terminology issues here.
The SDK is not closed source, you can find the source here: https://github.com/bitwarden/sdk
It might not be GPL open-source, but it is not closed either.
Other than that, I agree with your points. I don't agree with the kneejerk hysteria from many of the comments - it's one of the worst things about FOSS is how quick people are to anger (I am not referring to you here).
But all of that still doesn’t explain what their goal of introducing the proprietary SDK is.
Let's wait and see before we get out the pitchforks.