The ASUS's published specifications also say Non-ECC for the other boards you linked on geizhals.eu, for example: https://www.asus.com/us/Motherboards/PRIME-B350M-A/specifications
Also, there is some speculation that any implied support for ECC memory really just means that a stick of ECC RAM will work, but without the error correction, in which case that extra bank would be dead weight and a waste of money: https://community.amd.com/thread/210870
So far I'm not convinced either way.
EDIT: more: https://www.reddit.com/r/Amd/comments/5vlg7o/asus_updated_its_website_and_removed_all_mentions
"Convinced"? Either you want it or not, either its implemented or not :)
There is only one sure way to find out - check handbook. If there is ECC menu in handbook, then its supported. Otherwise ECC function of unbuffered will be ignored, just as you wrote.
But ASUS is probably the only definite vendor for unbuffered ECC in desktop AMD motherboards...
If CPU supports ECC, ASUS usually provides the needed gluing in BIOS/UEFI to make it truly work. Other manufacturers will just cut out the menu, but not ASUS.
The big problem is that some AMD CPUs do not provide ECC, here also ASUS board does not provide it.
If ASUS does not support it, but it is supported by AMD cpu, this will be big surprise...
AMD+ASUS is also the best and cheapest way for normal user get ECC advantages, server motherboards are quite bad for regular user.. a lot of unneeded functions, a lot of special cases. Like no sound, useless onboard VGA, 3.3 volt only PCI, little expansion slots etc.
Lets wait for handbooks...