“Works fine” just means the bottlenecks aren’t obvious in his workload, not that they don’t exist.I have an x370 board I gave to a friend that's rocking a5900x just fine. Most people won't notice the older pcie bus speed loss for the most part, as ssd speed difference don't matter as much for most programs.
It doesn’t mean it’s equivalent, and this is where AGESA matters. Newer CPUs on older boards rely on back ported AGESA, which is almost always a cut down version. Vendors strip features to fit ROM size limits and to maintain basic stability.
AGESA on X370 is a compatibility backport with conservative limits, like dropping a Ferrari engine into a VW...