Platform is normally the term that would be used. Yes, NF authors are usually expected to have it. It's a measure of what sort of built-in audience you already have via being recognized as an expert in your field. I know an author whose publisher stopped publishing her after platform became a thing. They said, "We can't publish you any more on this subject. You don't have letters behind your name. You don't have a proven public following." In other words, there was a shift away from generalist writers researching a subject and writing about it to experts writing about the subject themselves, preferably already having an audience through a blog, show, public office, pulpit, etc. But this was writing for adults.
It does depend on the type of books. As has been said, NF PBs haven't required this, nor has children's NF work for hire. I don't know if things are changing for MG and older with regard to trade NF.