“indicates”… that’s not what you’re trying to convey, is borderline misleading and how you get that to evidence is beyond me. This article suggests that it’s not the case, ASRock Rack X570D4U-2L2T Review an AMD Ryzen Server Motherboard - Page 3 of 4 having a quick search and there a lot of forums posts in general which questions “working” support being unconfirmed.
The older X470 does however appear to have proper support: ASRock Rack has created the first AM4 socket server boards, X470D4U, X470D4U2-2T - #1629 by Apple2c
I’m not sure what you’re trying to achieve because you seem to completely miss the point about why you’d want ECC memory in the first place and secondly have you even tried applying any of your overclocking rants this in real life and not in theory? If so, please share what hardware you used, data etc. Working support including error reporting which people contrary to your belief actually monitors is useful. If working it will show up in the system log on your BSD/Linux system which is what most people will use here and even tell so which also is likely in this case given Which flavour for home-media server for Linux-beginner? .
Yes, I should add “recommended JEDEC specs” which is what everyone else refers to since you seems to deliberately misinterpret it.