Apparently the UVA researchers relied entirely on serologic immunity, and did not look at the memory B cells, CD4+ T cells, and CD8+ T cells. Thus, their conclusion cannot relate to the entire immune system, and cannot be valid for immunity overall.
A more complete picture of immunity is in Science Magazine (link below). The conclusion is:
"Substantial immune memory is generated after COVID-19, involving all four major types of immune memory. About 95% of subjects retained immune memory at ~6 months after infection. Circulating antibody titers were not predictive of T cell memory. Thus, simple serological tests for SARS-CoV-2 antibodies do not reflect the richness and durability of immune memory to SARS-CoV-2."