Apple disagrees, and I would take Apple's advice over some random anecdote. There is no reason that an external photo library would be inherently worse than an internal one.
If that person actually did use a hard drive (not an NVMe SSD), then yes, the experience would probably be janky. I don't think modern Apple Photos is built to handle slow spinning rust very well.
If someone uses an external SSD with equal or greater performance to the internal SSD, then it would naturally perform as well or better than the internal photo library would have. Apple was putting a ~3GB/s SSD inside the M1 Mac Mini, which is hardly cutting edge and can be matched by an external thunderbolt NVMe enclosure. $120 for the enclosure + $70 for the SSD is still way less than the $400 that Apple is charging, and it can actually be upgraded down the line. Or someone could get a 2TB SSD for $130 (+ $120 for the enclosure). Apple wants $800 for 2TB of storage!
Even if Apple upgraded to faster SSDs for the new M2 Mac Mini (which is not clear at all), 3GB/s would still perform exactly as well as it did on the M1 Mac mini's internal SSD, which is to say... it would perform plenty well.
