Your link doesn't address better protection against variants specifically, just better protection against reinfection in general. The linked study specifically states that it does not include delta.
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
Getting a vaccine after recovering from covid would be along the lines of getting a booster shot. You'd develop some new antibodies, and strengthen your immune memory for the disease. It'll offer you better protection no matter what.