The artist is indeed loosing the choice to just monetize the content he wants, and he indeed loses the ability to monetize content on youtube without being bound to release on Youtube first/at the same time. nosequels gist is correct: the artist has now less choice than before.
I don't think that's a fair summary of the original comment, which also asserted that the artist is losing the choice to enforce their copyright unless they join the new program.
Personally I don't really like these changes, but let's not be disingenuous.
That does, however, suggest to me an option (c) that Google isn't offering: Musicians pay Google to run Content Id on their behalf to simplify the takedown process. While I'm sure there are some win-wins they're missing by not offering that (possibly including Zoë Keating), I really can't fault them for that. I expect the PR and legal complications would dwarf the storm that's already here.
Publicly available yes, open source no. You're free to choose whatever license you want:
You're under no obligation to choose a license. It's your right not to include one with your code or project, but please be aware of the implications. Generally speaking, the absence of a license means that the default copyright laws apply. This means that you retain all rights to your source code and that nobody else may reproduce, distribute, or create derivative works from your work.
It is now only publicly available
If Github did this, they would lose everyone overnight.