The thing that bothers me about "nonbinary parenting" is that parents are so often unaware of all the subtle social and nonverbal signals they still give off to gender their kids.
In the 80's and 90's, "gender neutral parenting" was a bit of a trend, especially among college-educated liberal parents. But it pretty much just amounted to neutral-colored bedrooms and giving the kid a few androgynous toys like Legos. Nothing much deeper than that.
Those "gender neutral" kids still picked up on gendered trends super fast -- they noticed Mom was more likely to be the one doing the dishes and laundry, and that Dad sat and stood in a way that took up more space, etc. They noticed that teachers, family friends, and neighbors treated them in gendered ways. When the kids picked up on and replicated gendered expectations, their "gender neutral" parents often threw their hands up and claimed gender stereotypes were actually true. It was so frustrating to witness! And I think no matter how well intentioned, nonbinary parenting is gonna be much of the same thing.
I don't know what the answer is, but I do know that raising a kid in a way that's free of noxious gendered messaging is impossible. I think it's admirable to try and resist the coercive nature of assigning gender at birth, but I think some cis parents who set out to do that don't really think about how deep that fight goes.