> since they have some of the most experienced lawyers there are when it comes to defending fair-use/free-as-in-freedom

And why would one assume that Github or MS do not have such experts? They undoubtedly have the technical know-how, and the primary findings in the letter are of a technical nature, or even obvious to technically savvy people. And the court decisions referred to are not about fair use or free-as-in-freedom.

I agree that they were obvious and, as you say, the HN conversations show that they occurred to many technologists. That said, I think there is an argument to say that the EFF was better qualified to write the letter. The reason being that MSFT wants to look like an impartial content host (to avoid being liable) and the EFF is explicitly an advocacy group. If MSFT advocates for content on that platform, it could be portrayed as a conflict of interest by the RIAA lawyers. I completely understand the optics of EFF doing the heavy lifting on this one.