As the one who started this thread, I'd like to make an observation.
Although I am scrupulously following the stay-at-home order, I see many people who have moved past it.
Yet, there is no new outbreak.
It makes me believe that people who have it do have immunity for some time.
In my case, perhaps it just never really left and flared up again later.
While I’m not disagreeing with your premise that it seems immunity (at least to some extent) exists, there are many reasons that you would not be seeing a new large outbreak yet beyond immunity. This holds true in places everywhere around the world that have gone through this, including places where there has been a very small percentage of the population infected.
Some possibilities:
-There are far fewer infected people around now than before
-As much as “people have moved past it”, the amount of mingling between and beyond social circles is very very far from where we were previously. Think shuls, Yeshivos, stores, weddings. I know of someone who broke all the rules last week and had an 80 person indoor wedding. Compare that with the smallest typical wedding in Bais Faiga, which has a minimum of 350 people invited to the meal alone, which goes on in multiple places nightly. People keep certain standards of distancing everywhere, and there is still a lot of mask wearing and hand washing. This has a tremendous impact.
-Testing and awareness are way up.
-It is way too early to see an outbreak, as each passing day more and more people are getting lax. A week or two ago most people were still being much stricter than they currently are.
In summation, while I do believe (some) immunity is a thing and I think many people in my community have it, to arrive at that conclusion from your observation is unscientific to say the least.