Notice that this is for common coronaviruses, and doesn't mention SARS or MERS much: IIRC, the former elicited an IgG response years after exposure in an experiment. As far as I can tell, this preprint does not cite that work.
MERS had a much, much higher fatality rate (~35%) and infected a small handful of people (~2500) so it's harder to get meaningful data.
>Two findings emerge: first, the T cells generated back then are still active 17 years on. And second, those old T cells offer protection against the new coronavirus. https://www.telegraph.co.uk/health-fitness/body/no-antibodie...