Whether it's social distancing or some other factor, the important point here I think is that it wouldn't have died down on its own. For you to look at the positive outcome alone and say "see, it wasn't a problem to begin with!" is just as irrefutable as to say it was the social distancing. So you have to use understanding of the disease, logic, data from other countries, etc, to come to your conclusion.

(You ask an annoying, but fair and important question here. We shouldn't lose our skeptical mind.)

The claim is not that if it dies down after social distancing, therefore social distancing was to blame, but that if it were to _really_ die down due to it, the measures would seem like an over-reaction to a certain segment of the populace. I think that, while a bit presumptuous, will hold true.

Given the different dates (and levels of aggressiveness) countries are enforcing social distancing, it should be quite refutable.