> there's something else going on: either it's way more infectious (but less dangerous) than realised, or more likely, a very large proportion of people have baseline immunity to the infection. It spreads rapidly through those who had no previous immunity and then burns out.

I don't think this can be the explanation, because it would not account for how geographically localized it was in Sweden. The Stockholm metro area got hit heavily, but the rest of the country was comparatively spared, and it never became big in southern Sweden. If people had prexisting immunity to coronaviruses you'd expect the immunity to be equally distributed, so it should burn through the entire country.

To me, that sounds more like there was an outbreak that started in Stockholm, but then they added social distancing measures which were enough to slow it down...