Source code files are relatively small and modern OSes are very good at caching. I ran out of SSD on my build server a while ago and had to use a mechanical HDD. To my surprise, it didn’t impact build times as much as I thought it would.
It's caches all the way down.
I don't think many 8-way Xeon Platinum boxes have eMMC storage.
I believe that this makes sense:
In a typical C++ project like that which use template libraries like CGAL, compilation of a single file takes up to 30 seconds of CPU-only time. Even though each file (thanks to lack of sensible module system) churns through 500 MB of raw text includes, that's not a lot over 30 seconds, and the includes are usually from the same files, which means they are in the OS buffer cache anyway so they are read from RAM.
However, if the project uses C++ like C, compilation is a lot faster; e.g. for Linux kernel C builds, files can scroll by faster than a spinning disk seek time would allow.