From the Long Now article:

The fast-and-stealthy scenario is consistent with the evidence reported to date from China, Italy, and isolated cruise ships. Consider the Diamond Princess as an extreme example of what can happen with an older population and delayed social distancing: 3,711 people on board, about 700 had the virus when tested, 400 of these were asymptomatic, 300 became sick, 7 died. But, how many of the 3,711 had recovered before they were tested? One of the problems is that the current tests for COVID-19 only tell us if someone is currently hosting the virus. The vast majority of people tested so far have been negative; is that because they never had the infection or is it because they had already successfully recovered? If 3,000 on the Diamond Princess recovered before testing, the mortality rate was 0.2%. That is high, but it is much better than the mortality rate of those we know caught the virus.

This seems very unlikely to me. For this to be true, the majority of humans would have to be able to develop antibodies and then clear the virus much faster than a substantial minority, and the virus would have to spread much faster than we currently think (i.e., have a much bigger R0 or shorter incubation time which would it place it outside the range of other respiratory viruses). Is there any inside view or outside view reasons to think this? E.g., has there ever been another virus like what the Long Now article suggests?

To distinguish between the most optimistic and pessimistic possibilities, we would need to measure how many people have developed an immunity to the virus. Fortunately, there are at least two different well-understood methods for developing such “serologic” tests, and the first such test has recently been developed for COVID-19. The first studies to use these tests are already underway in China and Singapore. We should know the results within a few months, if not weeks.

It seems not very decision-relevant for the vast majority of LWers to bring up this "optimistic possibility" at this point, given how unlikely it is, and how long we'd have to wait to distinguish it from the the "pessimistic possibility". (Also I don't like the subtext here subtly suggesting that until we know for sure, the two possibilities should be equally salient.) Am I missing anything, or is decision-relevance not the main reason for linking this?