The Oxford vaccine (ChAdOx1) uses another vector virus called the adenovirus to deliver the coronavirus genes which the body can fight against. The issue is that some people might have pre-existing immunity to the adenovirus, making the vaccine ineffective. Since ChAdOx1 uses an adenovirus which infects chimpanzees, it is likely that most people will not have any pre-existing immunity to the vector virus. I guess the parent comment is referring to the fact that the body might develop immunity to the chimpanzee vector virus after two doses, making booster shots ineffective.