60k kids have avoided peanut allergies due to 2015 advice, study finds

I wonder why the old advice was being given if it was so wrong? If nobody understood what to do, shouldn't there have been no advice instead of something harmful?

You seem to be suggesting that doctors should not suggest any health precautions until controlled experiments have found them effective. That is the position taken by the highly-cited paper "Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomised controlled trials", which you must read immediately, because in a peculiar way it is a paper about you: https://pmc.ncbi.nlm.nih.gov/articles/PMC300808/

You don't need a controlled experiment if you have a good enough understanding of the mechanism, such as with parachutes. But since they apparently had no idea how peanut allergies worked nor had any adequate studies, they should have just shrugged their shoulders when asked for advice.

Even with parachutes, you could do a study (not a RCT) by looking at historical cases of people falling with and without parachutes. The effect would be so strong that you wouldn't need those clever statistical tricks to tease it out.