Update: SRPMs are now available in https://vault.centos.org/centos/8-stream/AppStream/Source/SPackages/
The SRPMs aren't currently available.
According to Johnny Hughes on the CentOS mailing list (17 Dec 2020):
I'm sure src rpms will also be available in the future, once all the
hardware and mirrors are in place (we can not support downloading
directl off ...