They aren’t claiming the SARS antigens prevent Covid. Their T-cell immunity prevents it. Just like it certainly will with the variants.
This will be one more thing like masks, seasonality, child risk, outdoor spread, vitamin D, etc.....where the official narrative is one thing, people using common sense disagree with that narrative and are mocked, silenced, censored and then eventually the people who control the narrative say, “we weren’t wrong....now that we have more information, the science has just changed”
Memory T lymphocytes are still antigen-specific. The CD4+ and CD8+ T cells require an MHC receptor bind site. That bind site requires specific molecular components to function.
NK T cell lymphocytes are not antigen-specific, but they’re also not altered by previous infections. Ergo, no memory.
If you want to claim there’s some other mechanism for T cell memory which doesn’t involve antigen-specific MHC binding, you’ll need to identify what exactly that is because I’m not familiar with it. Otherwise, antigenic shift and drift are problems for T cells the same as they are for B cell produced neutralizing antibodies.