I give it a 51% chance of Discovery Loop being acquired by Google

I actually think there's a low probability of that.

The four have so much influence within the company that they could have trivially set this up as part of Alphabet, if they wanted to. They are also ridiculously wealthy.

They are already partially funded by Google.

I feel it's image management and a bet. If they succeed, Google profits. If they don't succeed it doesn't matter, it's still a signal to the market/wallstreet that there's no "bad blood" between them and Google and that they won't be cannibalizing Google's current interest.