This is full ECC, the CPU supports it (AMD Pro variant).

DDR5 ECC is not good enough. What if you have faulty RAM and ECC is constantly correcting it without you knowing it? There's no value in that. You need the OS to be informed so that you are aware of it. It also does not protect errors which occur between the RAM and the CPU.

This is similar to HDDs using ECC. Without SMART you'd have a problem, but part of SMART is that it allows you to get a count of ECC-corrected errors so that you can be aware of the state of the drive.

True ECC takes the role of SMART in regards of RAM, it's just that it only reports that: ECC-corrected errors.

On a NAS, where you likely store important data, true ECC does add value.