it says "A more lively project; still, definitely targets systems with a real Linux kernel." but actually people are using Musl for other use cases. And it is BSD licensed, and very high quality.
Okay, what good is any of that if musl literally does not compile without Linux headers?
This is for systems that are at best a small RTOS, and there is no standard for what those operating systems offer to a libc.
Musl upstream only supports Linux, but it is so easy to port that there are large numbers of ports to non Linux environments.
There were other considerations against musl there. I'd summarize them as targeting a desktop/server with kernel level system, or at least a mid-to-high end embedded system, has different code, memory, and optimization tradeoffs than the smaller embedded systems that seem to be targeted with picolibc.