No sarcasm, Bionic libc is a subset of POSIX, Musl strictly follows POSIX 2008 and Glibc is superset of POSIX. 99% of Glibc->Musl porting is dealing with non-POSIX features (a.k.a. GNU extensions)[1]. So in the end we could find that Android apps are more POSIX compliant than software from generic glibc-based distros.
Where are the posix utilities ? /s Where is man ? /s