> May be such infusion would really serve as a vaccine prepping the immune system for the main infusion later?

This is correct and exactly the problem. You don't want the immune system to react to the virus, but giving it exposure to the virus will nearly guarantee a reaction. This is why they were checking that the patient didn't have antibodies already.

Here is an article discussing this issue: https://pmc.ncbi.nlm.nih.gov/articles/PMC10673641/