It's important to note that this is them moving in-development branches/features "behind closed doors", not making Android closed source. Whenever a feature is ready they then merge it publicly. I know this community tends to be filled with purists, many of whom are well informed and reasoned, but I'm actually totally fine with this change. This kind of structure isn't crazy uncommon, and I imagine it's mainly an effort to stop tech journalists analysing random in-progress features for an article. Personally, I wouldn't want to develop code with that kind of pressure.
Lots of people make a PR very early though, just to keep track of development and have a space to jot down thoughts and ideas, and get feedback during.
Heck, I'll sometimes make a wip.diff file and scp it back and forth between work and home machines just because the code feels not ready for other eyes.
Would you really want everyone in the world looking at every end of day commit before you've refactored it into something vaguely passable?
Honestly, it has been fine. Almost nobody really pays attention to anything they don't care about, and most people who do care tend to be pretty helpful.
Not only that, the Android Police article mentions they had a lot of trouble merging the internal branches and the public branches, so I’m guessing as time went on they’ve diverged more and more.