So a more more accurate description seems to be "there is no evidence that thinking of the viruses in terms of multiple strains is scientifically helpful".
More seriously, I get your point but many discoveries are done by finally observing something which existence has been anticipated, hypothesized but still unproven. A planet that had an effect on nearby objects because of its gravity for instance. Or the Higgs boson.
So, yes, thinking of things that are not proven to exist (yet) is often scientifically helpful.
You still want to apply Occam's razor of course.
Is that clearer?