Exactly. I don't think Google is doing this so that people don't install some random FOSS alternatives through F-Droid.
Things like Newpipe seems much more of a target, especially if you want to take legal action. More so than stopping users, this gives Google fat more leverage about what Apps can exist. If they ever want to stop Newpipe a serious lawsuit against whoever signed the APK seems like an effective way to shut down the whole project. Certainly more effective then a constant battle between constraining them and them finding ways to circumvent the constraints.