well one of the stated goals of musl is to be simple and correct, and all those mallocs are anything but simple
I have to lookup what exactly mimalloc is/does every time someone mentions it, because the readme/documentation isn't very descriptive except discussing extensions outside the normal API. I didn't have time to dig through this again today. But I did look at it in some depth on several occasions in the past and it really wasn't suitable for or comparable to what we're doing in musl.
You should definitely have a look at the paper [1] it's only ten page long! (Excluding benchmarks and references)
[1]: https://www.microsoft.com/en-us/research/uploads/prod/2019/0...