I'm going to have a go at being
@Campbell. If I'm wrong he'll tell us both!
In another recent thread the distinction was drawn between "winging it" - as in, the GM makes stuff up as s/he goes along but with the status and purpose of that stuff being functionally comparable to pre-prepared notes - and "no myth story now" where the function of the stuff the GM makes up is different, because made up according to different principles which put the focus on
these characters in this situation facing this challenge to their own needs and desires.
I'm guessing that Campbell sees MotW as more like the first - I'm going to guess with robust mechanics to support the improv - rather than the second. Cthulhu Dark could probably be played like that too.