That's looking at relative risk not overall. It's not saying you lose immunity after 180 days.

Natural immunity wanes enough to cause a "2nd surge" in cities that were 75%ish infected, such as Araraquara city, Brazil, and Manaus, Brazil.

I can agree with you that the 180-days thing is just an estimate from one study. Further studies are needed to narrow down the exact time when natural immunity wanes. But its a good "first guess" at the phenomenon, and should be within a magnitude of the truth.

From what we've seen around the world, COVID19 (or at least its variants, like Gamma/P.1 or Delta/B.1.617) is more than willing to reinfect populations in less than a year.

The Manaus study used a convenience sample, meaning that you cannot infer population rates from it.

Who would have thought that reading the abstract of a paper could be a super power?