“ASRock” and “ASRock Rack” are two different series of motherboards.

Here’s the QVL of one of their AM5 mainboards: https://www.asrockrack.com/general/productdetail.asp?Model=B650D4U-2L2T/BCM#Memory - it doesn’t limit these modules to specific CPUs. All CPUs with ECC compatibility also support these modules on this mainboard. Some of these Rack boards are over a year old, and they always had some ECC modules on their QVL. This - again - isn’t EPYC 4004 specific, they couldn’t have validated it with EPYC 4004 CPUs a year ago. In fact, their CPU support list doesn’t even list EPYC 4004 CPUs as of today, as they haven’t released a BIOS update adding (official) compatibility in for these CPUs (it will probably be released shortly though).

ASRock Rack AM4 mainboards also officially support ECC memory. So if you wanted verified ECC support on a comparatively cheap AMD platform you could’ve always gone for one of these boards with a regular Ryzen CPU (not an APU). The boards are a bit on the expensive side but if you want official support (for whatever reason you’d need that in a homelab environment) you can get it.