The EXARCH dependency checking is as of now optional since it is really slow. The key issue is that has bee n hard to find the source rpm used to build a specific dependency.

Now, dnf supports queries like "dnf repoquery --disablerepo='*' --enablerepo=rawhide --whatrequires libconfuse --source --alldeps --available". Using this it should be possible to re-implement this in a more sane fashion.