> Depending on how that case finally comes back, I'd expect that could really affect Microsoft's strategy, here
Remember that because there were originally patent claims in the suit, the appeal went to the Federal Circuit -- even though the patent claims were dropped -- not the Ninth Circuit where the district court is located. As a result, the Federal Circuit's ruling that APIs are not uncopyrightable is not precedent since it doesn't concern patents (the Federal Circuit's remit).
Their reasoning might influence future rulings, but precedence-wise the ruling has no bearing on the future of copyright on APIs in the US.
The way I see it, this is an open source shim layer. I don't see any copyright issue whatsoever. Whoever chooses to use the shim is using ffmpeg anyway, so they are already exposed to the license in exactly the same way. This just makes it easier to use ffmpeg 'the right way' on various devices, if I understand correctly.
If somehow MS can't make and open source clean compatibility layers for their own software, then that's a big problem.
I think you're mistaken. This project does not "implement and API written by someone else". They implement a Windows API interface (MediaStreamSource) that wraps the FFmpeg library so that it can be easily used in various Windows OSs.
Depending on how that case finally comes back, I'd expect that could really affect Microsoft's strategy, here.