Most of musl's performance issues come from their allocator. Using it with a third party high performance allocator allows you to benefit from static linking with very little performance loss.
This is addressed and disputed very early in the article. The very first benchmark presented shows a 26% regression using musl + mimalloc, a high-performance 3rd party allocator.
And the compounding issue is that the allocator issues get significantly worse as parallelism increases, as the allocator is serial, so as concurrency increases so does the impact of the allocator, which is not the case for most of the "regular slow" code (of musl), those have a relatively constant overhead per thread.