IMO, "validated" is intentionally wishy-washy and mostly means that AMD would prefer it if enterprises paid them more money by buying EPYC (or Ryzen Pro) parts instead of consumer Ryzen parts. Much like how Intel prefers selling higher-margin Xeons over Core i5. It's market segmentation, but friendlier to consumers than Intel's approach.

More like "The feature is present in silicon but motherboard makers are not required to turn it on". At the end of the day, ECC support does require extra copper traces in the PCB and some low end models may deliberately choose to skip them, thus the expectation has to be managed.

My (largely unfounded) understanding is that this means they don't don't run the consumer chip configurations though a battery of compatibility tests with with various memory modules on a reference motherboard. My understanding is that they run these steps for each stepping or significant process change for their.

Secondly, it probably also means that they do not include tests for this functionality when they perform the final tests against each fully assembled chip. I'd expect that a jtag boundary scan does verify that the bond wires are in place and work, but no functional tests of ECC are run on each processor in the consumer configuration.

The net result is that with a compatible motherboard and memory, ECC almost certainly works (since the memory controller is the same as in the supported model) but AMD does not officially guarantee it. It is much like overclocking. The functionality is present, and it should work, and most likely does, but AMD accepts no responsibility if it does not, since they don't formally test for it.