> a similarly minimalistic allocator
mimalloc has about 10kloc, while (assuming I'm looking in the right place) the new musl allocator has 891 and the old musl allocator has 518 lines of code. I wouldn't call an order of magnitude difference in line count 'similar'.
It's minimalistic in the sense that it compiles to a tiny binary (a lot of the code is either per platform, musl is POSIX only afaik) or for debugging. Yes it's bigger, but still tiny compared to something like jemalloc, and I'm sure it's like 10kb in a binary.