I think they're saying that we can hope that three shots gives long term resistance, as it happens for other vaccines. We can only hope, and it's not a totally unrealistic hope given how other vaccines work.
https://www.cdc.gov/vaccines/schedules/hcp/imz/child-adolesc...
The flu, for example as existed longer than COVID and we do not have a "3 dose" vaccine that prevents the flu for a decade. Why would we expect this for COVID as opposed to an annual booster?