"Many devs choose Android (over Apple) specifically because it's open-source."
Some do. Some select their platforms based on the developer tools. And some select the platform based on maximizing revenue, user base or just goodwill.
Choosing Android because it is open source is kind of a problematic point since most Android phones (>99.9% ?) have the Google core software installed which is not open at all. Not to mention the drivers which AFAIK are usually just closed binary blobs.
If openness was a major factor for devs then Firefox and Debian phones would have had a huge app market. That didn't happen.