They did provide protection against infection initially, and did so very effectively.
It was not known at the time whether that protection would maintain or not, nobody knew how the virus would mutate.
It was not known at the time whether that protection would maintain or not, nobody knew how the virus would mutate.
Claims that the vaccine provided immunity were always baseless. What the trials demonstrated was that the vaccine was effective at suppressing symptoms and hospitalisations.
And this is not something that came after the fact, I remember that this approach was discussed on HN when the results were initially published.
Even if there were a minor correlation, then you can infer some information from the trials.
I'm not also suggesting that what you describe isn't part of the story, but there is no reason to believe it's a major part of the story. Viral infections lead to symptoms (most of the time). The vaccine is not a magic totem that allows for massive amounts of viral replication while just taking away the symptoms. That isn't how the immune system works.