It's tough to make an intuitive story for why RCTs are important, so the story sounds elaborate. You don't need to offer evidence against the theory to say that the methodology is weak.
Anyways, wet streets might cause rain.
And how do you produce a social placebo to detect such effects by comparison? No treatment is not the same as known fake treatment.
In fact, even the observational study here is quite underpowered and not multicultural. It is to be taken with a few bags of salt.
If a hypothesis leads you to test for something and collect (dis)confirming evidence, And your argument hinges on the evidence, then we aren’t arguing from ignorance.
A good argument against a study's methodology would probably talk a lot more about the methodology, and a lot less about a believable alternative narrative that is supported by the study. The latter strategy is accepting of the methodology because it is granting that the observed effect is real and deserving of explanation. In a different light, the earlier comment ("As an example of how the observed differences could still be due to nurture rather than nature...") is kind of weird because it is as though the effect might as well be due to nature -- it would present the same way and be as consistent and robust -- even if it were due to socialisation.