Maybe not helpful, but I added this repo to my Nexus Repository Manager and was able to list all the packages, but from a CentOS 7.7 installation. I don’t have a CentOS 8 install readily available right now.
I’m on nxrm 3.22.1 and used
https://dl.fedoraproject.org/pub/epel/8/Modular/x86_64
or
https://dl.fedoraproject.org/pub/epel/8/Modular/x86_64/packages
for my URL. I tried both, but I’m not sure which one is working because I’m not sure if the repo data is cached when/if I’m using the wrong URL. I can do further testing, if you can’t get one of the URLs working still.
I could also install CentOS 8 and test, if you think it would be helpful.