Not that this validates the claim, but this was a pretty influential post by the author of Flask circa 2016:
https://lucumr.pocoo.org/2016/10/30/i-dont-understand-asynci...
https://lucumr.pocoo.org/2016/10/30/i-dont-understand-asynci...
I haven't read the post in detail though, thanks for the link! Seems interesting.