Hi. It's not that the CMBR definitely isn't the afterglow of the Big Bang. It's just that this conclusion was rushed to on the basis of some poor science. This is never discussed in the official narrative, so my complaint is that it should be explained why the alternative, and apparently superior, theory is incorrect, and that Big Bang is better. For details see:

https://graham-pemberton.medium.com/did-the-universe-begin-with-a-big-bang-part-2-possibly-probably-not-d4d492959f8d