I would say the main point of the study is that the variant in circulation now is sufficiently different than the old vaccine that giving such a failing vaccine to old people as a third shot is throwing away a shot that might protect 30% of those who get it who were never vaccinated.
Of course, this practically means that the older person not getting the third dose is likely to have a really bad infection, if that third shot was truly protective for at least 12 days. Older people need much stronger doses of vaccines for them to work, as we know from special flu shots for the elderly.
It appears the booster mitigates hospitalizations and deaths concerning the delta variant in breakthrough cases from the abbreviated study. Whether that will carry through long term remains to be seen.