OK, so here's the basic issue.

What did banning third-party cookies actually stop then? Nothing?

So whatever they were able to do before banning third-party cookies, why did banning third-party cookies matter at all?

The whole point of the CNAME as described is to get around third-party cookie bans. If getting around the third-party cookie ban doesn't actually help -- that seems to suggest the third-party cookie ban in the first place didn't actually do anything, if getting around it doesn't give you anything? no?