It is strange that they didn’t at least use a Mac mini, but I assume they already had the hardware.
Back in the early 00s I hosted websites on my local off the shelf Dell desktop on a cable internet connection.
Cloud computing wasn't really a thing and I didn't know better / have much better options (eventually I did migrate to some webhost bundled with the DNS registar).
For the longest time network bandwidth was my limiting factor. I remember finally upgrading my connection, NIC, etc. to the point where it was actually harddrive write speed that was the bottleneck.
Probably depends on your application but harddrive I/O does seem like a likely limiting factor to me today.
Also save for the fact that commodity hardware lacks a lot of the hardware safeties a server does. For instance, your m.2 under sustained load would not perform at the same rate that an enterprise server hard drive can; we're talking on an order of 180MiB/s vs 550MiB/s per drive. Once the m.2's cache is exhausted it will be comparable to a spinning disk in performance.