It's the upstream source, not the source used to build RHEL. RHEL is downstream of CentOS stream. Much like linux kernel's source is available, it does not help you much.
According to Red Hat it is. If you don't believe them you can get a Developer subscription to get a RHEL ISO to compare with a CentOS Stream ISO. I imagine a lot of people, myself included, would be interested in the analysis of that.
So you do that analysis. Two downstream distros of RHEL have said that they cannot continue to offer RHEL rebuilds/updates without substantial changes. That means that it is not possible to use CentOS stream alone to build RHEL packages. You are free to do all the analysis in the world.
It may incidentally be in some cases, but isn't guaranteed to be. Things don't necessarily flow back up from RHEL to CentOS stream, and even if they do in a general sense, it may not be sufficient to build the exact same tag that RHEL uses. Someone else mentioned that they were not able to build RHEL packages from CentOS stream.
Pushing things back to CentOS stream is not a requirement of GPL. GPL's role ends once RHEL customers get the source. Do you mean evidence of CentOS stream not being enough ? The post talks about it. Rocky needs to collate sources from several places now to create 1:1 RHEL rebuilds.