The BBS show "More or Less" discussed this study.
There is a lot to sort out, but I'll try to summarize as accurately as I can plus some subsequent reading.
Antibodies have a half life. For measles, it's 200 years. For tetanus it is 11 years. It's looking like for Covid it may only be a few months.
But the immunological response depends on a combination of antibodies and memory B cells that can produce more antibodies. So it's not clear that antibody levels falling to "naive" levels is the same as the immunological response returning to naive levels.
Measures of antibody levels is merely a proxy. The best indicators will be infection rates, hospitalization rates and death rates. It seems like the vaccine is not holding up that well against simple infection, but is still holding up well against severe illness, perhaps because the memory B cells have that memory and can ramp up to produce antibodies faster than in a naive patient.
What would be most helpful is epidemiological studies.
>> booster
I'm ambivalent about this. The head of the WHO has likened this to throwing out a second life preserver to someone who's life preserver is getting frayed while there are others who still have no flotation device at all.
And if it's true that the life preserver is already waterlogged and starting to sink, it still raises the question of how many life preservers we throw at rich mostly healthy people when high-risk poor people are still waiting for one.