musl is a libc, and while it is superior in some ways, it is inferior in others. If you want a statically linked libc, or a permissively licensed libc, musl is a fantastic choice. If you want a libc with the fastest malloc implementation, you'll probably want to look elsewhere.

Can’t you sub in your own malloc like the one in the article or jemalloc if you don’t like the performance of the original in your app?

Sure, and lots of projects do. I was just trying to make the point that there isn't one "good" libc that is universally better – each project has costs and benefits that are tied to its priorities.