It as only a “bad” strain because there was a massively immunologically naive population exposed to it.

Yeah no it don't think it was, maybe "strain" was the wrong word here. Death rates in Europe were also much higher than usual, so there was definitely some major mutation going on. Naive populations will certainly have played a part, but I still think the virus gaining a captive host population to mutate in for several generations on each of those troop transporters, before those incubated populations were then spread all over the country, had an effect on it being more deadly in America than it was in Europe. I mean even the article you linked seems to suggest as much:

Military transport ships were the likely vector of influenza which was well-established around the world by August of 1918. As the pandemic grew and matured its virulence apparently increased. Mortality rates on the eastern coast of America climbed in newspaper reports, the epidemic seeming to emanate from military bases there. Thus, what had been called the “three day flu” at Camp Funston in March, with a mortality rate of perhaps two-percent, evolved into a much more severe illness in India where mortality rates in some places may have reached ten-percent.