Probably they said after three months antibody levels dropped, just like people who had the actual disease. It doesn't say anything about immunity, because those people probably had activated memory T-cells that could get B-cells to crank out antibodies very quickly if exposed to corona virus antigen. Pretty normal. Since that is how people respond to the disease, I would not be surprised if all the vaccines work that way.

I guess the good safe test would be to expose them to a little corona virus antigen and watch their antibody levels over a few days.