So why are both musl and glibc from mid/early 2014, and dietlibc from 2009? That's an awful lot of cherry-picking versions with no stated justification.
I think it's because those are the freshest stable versions the author could use? uClibc 0.9.33.2 by itself was released in 2012. I guess he had a reason for using the BuildRoot 2015.2 version.