Seems like a horrible way to work. Why could this not have landed as a large amount of smaller PRs in a more incremental fashion, maybe Java users could have gotten some value out of it earlier.

Out of all factors this certainly isn't it. If anything, more projects could learn from the way Valhalla was handled.

It started out as a highly invasive, highly disruptive and highly experimental branch that eventually turned into a beautiful specification that put semantics and performance payoff first and syntax seconds.

This is incremental. This change is the smallest possible one that could work. There are more coming in later.