They were presumably referring to https://pubmed.ncbi.nlm.nih.gov/36548397/ https://pubmed.ncbi.nlm.nih.gov/36548397/ , where repeated vaccination with the mRNA

They were presumably referring to https://pubmed.ncbi.nlm.nih.gov/36548397/ https://pubmed.ncbi.nlm.nih.gov/36548397/ , where repeated vaccination with the mRNA vaccines was associated with antibody class switching towards IgG4 "tolerance" antibodies. In theory, this class switching could allow the spike proteins from the virus to persist longer in the body, causing more damage. The same phenomenon wasn't observed with non-mrna vaccines or covid infection, which has been hypothesized to be because they don't make the body's own cells produce the spike protein, so don't invoke that same immune tolerance mechanism.