I was surprised that there are very few PC cases made for small NAS. So this solution of buying a consumer product and installing FreeNAS on it makes sense.
That said, I'm surprised that they explicitly support installing Linux -- the post links a QNAP wiki page that explicitly uses the phrase "supported hardware" when describing which machines their guide to installing Debian works for. The case itself is pretty high quality for the purpose (no surprise), but I don't think I'd buy one just to wipe the custom OS and install my own on it. If it were me, I'd probably hop on over to r/DataHoarder, search there, and maybe make a "recommend me a case" post if I didn't find anything interesting.
I'd really love to do a NAS project with a Raspberry Pi, but the lack of SATA support really does it in as a base for a NAS system, IMO. If you're just going to connect everything over USB anyway, what's even the point of the Pi? Just buy one of those dumb external enclosures that holds 4+ drives.