It's not really true vs false, it's a matter of deciding what level of evidence is convincing. One point is that we already know the approximate effectiveness of social distancing from many 1918 data points and its epidemic with similar contagiousness. Some places in 1918 did it and some did not, like San Francisco vs Philadelphia where the difference was stark.

Maybe the effect will be less this time for some inexplicable reason but I would not want to run the experiment in my country.