Given that Google both owns Android/Google Play Store and YouTube: what do you think they would do with the developer information of someone who makes an app that skirts their ad-model for YouTube?
The "security" wording is the usual corpospeak - you can always trust "security" to mean "the security of our business model, of course, why are you asking?"
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.