No, it wasn’t ever. It always belonged to Google who benevolently open sourced parts of it and retains control.
Safetynet and PlayIntegrity are under Google’s control. The PlayStore is Google’s. All of the APIs are Google’s! Hardware blobs are closed and belong to the manufacturer.
Just because some of the stuff shows up on GitHub doesn’t make it an open platform.
Don’t overstretch my analogy like that. How about this:
Google has standardized The Restaurant. The kitchens all have the same tools and ingredients and are open to anyone. Seating and billing is standardized, and you can easily order and pay at your table, and the food is delivered straight to you via pneumatic tubes and nicely packaged. The food might be expensive or cheap, tasty or revolting, but the experience is always the same.
There are a lot of hobby cooks that like to cook in the Google restaurants. If you want to eat their food, you might have to pick it up straight at the kitchen, or nicely ask the hobby waiters. The cooks have been there for years and whipping up nice creations - mostly for free, beacause the ingredients and tools were free, and they really like to cook. Because the food is so good, some people tip the cooks or waiters directly.
Now Google introduced a new rule: everyone has to use their billing and pneumatic delivery system, citing improved food safety. The hobby cooks and waiters are infuriated, and even some of their customers, and they demand that everyone can still come to the kitchen or the waiters. But Google just says: look, my restaurant, my rules. If you don’t like it go make your own.