It does not say that there is non-zero benefit vs variants. This data does not exist, it could be the case that natural immunity is as good, better, or worse vs variants, or perhaps is different depending on variant.
it doesn't have to have non-zero benefit vs. variants, it just has to have non-zero benefit overall to outweigh the negligible cost in the net cost/benefit calculus
Keep in mind this discussion thread is about people who have already contracted Covid and recovered.
The benefits:
Unknown increase in immunity against alpha strain
Unknown increase in immunity against variant strains
...
The Costs:
Chance of currently known adverse effects
Chance of unknown adverse effects specific to vaccination of people with prior immunity.
Some recent data [1] suggests that those with prior covid infection 13X lower chances or reinfection with delta than those with 2 vaccine doses. If this is accurate, it begs the question of how much benefit is gained by vaccination on top of existing immunity in comparison to to the rare but known side effects.