Yeah, its not clear if preventing symptomatic infection is as crucial an end-point in contrast to preventing hospitalization, the latter of which is indeed mediated by T-cell immunity rather than antibody titers. The following detail is worth noting though ---

"Vaccine sera were collected from sixteen vaccinees four weeks after the second vaccination with mRNA-1273 (Moderna)"

So it seems like they haven't tested against sera from people who got three doses? Response to BA.1 was better with three doses than two, but I think if the data from Israel was to go by, the impact of the third dose on antibody titers also waned over time (although T-cell mediated immunity stayed the same in terms of % hospitalized).