"Helicopter" parenting could always point to experts and research saying that something was "good" for your child. In general, that's true, just like water is good for you. However, 10x good is not better. What slow parenting does is articulate why not giving something good to your child is also good.
1. Cultural activities for your child are good. It makes them smarter and better rounded.
2. Non-cultural activities for you child are good. It allows them to explore as they will, making them smarter and better rounded, and develops skills that #1 does not develop.
If we just went by #1, we would be temped to toss the cultural activities on because cultural activities are all good and there is no opportunity cost to doing them. However, when we add #2, we now have an opportunity cost. Too much of #1 loses us some of #2.
So now, having this in mind, the non helicopter parent can actually make a reasonable response to someone who is helicopter parent.