I agree that the OP makes a bunch of interesting points, but I think historically Brain really grew out of what Dean wanted to do and the fact that he wanted it to be full-stack, e.g. including the TPU. Also, crucially, Brain would use Google data and contribute back to Ads/Search directly versus Google X which was supposed to be more of an incubator.

But it's also notable how the perspective of an ex-Brain employee might differ from what sparked the Brain founders in the first place.