Interesting you mention the Diamond Princess, it actually supports the "far-fetched" theory. And it's not as far fetched as you think, it's gaining more ground and does explain places like New York, London, Sweden, India etc.

https://www.spectator.co.uk/article/immunity-to-coronavirus-...

"In the accidental experiment of the Diamond Princess – a close community where the infection was allowed to spread unchecked for a fortnight in January, and everyone was eventually tested – only 17 per cent of passengers and crew became infected."

New York (and similar UK and Sweden) has a higher death rate since they moved sick patients into care homes and decimated them. As you said everywhere saw similar curves no matter how strict or lax a lock down they had. Implying something else is going on and not the lock down.

Studies looking at the impacts of lock down on the curve are showing very little correlational https://www.thelancet.com/journals/eclinm/article/PIIS2589-5...

“full lockdowns and wide-spread COVID19 testing were not associated with reductions in the number of critical cases or overall mortality.”

So, to me at least, it does seem far more plausible that t-cells are more a mitigating factor then lock downs.

If I am cherry picking, seems to be lots of cherry trees to pick from.

I see no reason why t-cell should not offer some protection, like it does with other coronaviruses. And why we would assume that everyone is just as susceptible when we know the risk profile varies wildly.

EDIT: for your explaining your edit of "Life Care Center in Kirkland", the reason they have more antibodies is that we all know the older you get the less t-cell protection you have. A "life care" center one assumes is full of old people, hence lower t-cell protection. And it explains why children have more protection as their t-cell protection is stronger.