I never made that claim. Changes to the antigens of a pathogen (i.e. antigenic shift and antigenic drift) result in a diminished immune response. Hence why Influenza is a problem year after year even in people who've had the flu many times. This applies to any pathogen where antigens drift and/or shift. SARS-CoV-2 is no exception.
The number of people who actually had SARS2003 (7,322 worldwide) or MERS2012 (1,684 worldwide) is so vanishingly small that it would be difficult to accurately determine whether they have any immunity to SARS-CoV-2 without challenge trials. That would be unethical to do, particularly given the risks of ADE with the SARS-CoV-1 nucleocapsid protein antibodies. That's why those proteins were not targeted in any of the COVID-19 vaccines.
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”