I don't think they should be subjected to the mandate for this, because their immunity does last longer than vaccine-induced immunity. That has been shown in epidemiological studies, and also recently from the U.S. CDC data from California and New York, which showed that there was no benefit with respect to hospitalization for people who have had infection-acquired immunity getting vaccinated.
I know we've heard a lot about this hybrid super-immunity you get from being vaccinated even if you've had COVID, but that short-term spike in serum antibody levels is not really worth a mandate.