I mean Microsoft does own GitHub so I guess they can do whatever they want with their GitHub repos.

There is no requirement for anyone to use an open source license or even to have source code in a GitHub repository. It is of course expected by most people, but this is not a special privilege only granted to Microsoft.

I'm guessing the rationale of their producting marketing people is something like: Meet the developers where they hang out, etc.

I'd agree with these hypothetical "product marketing people".

As annoying as it is to deal with binaries over source I absolutely would rather deal with a git repo than just a handful of URLs.

Open source binaries?