There is a bit more to it. Chia "plots" are large files that act as bingo cards. You need an enterprise-grade SSD to generate a card but not to play bingo once you have it.
The files being expensive to create but not to reuse is what makes it proof of space, since it makes caching the data much cheaper than generating a new card.
Semantics aside, you require a SSD or better at some point and that was where Backblaze was seeing above average failures and started reining in Chia usage iirc
SSDs (or NVME) are used for plotting generation - but serving them can be done with big, dumb, slow spining drives.