as a marketer who sells courses myself, i'm either not qualified or extremely qualified to answer this question. ;)
but i think i understand the type of person you're describing. in short, someone whose primary marketing skills are selling courses about marketing sells that help you sell courses.
did i get that right?
many of these folks, unfortunately, are frauds. not because their content and claim to fame is circular (it is), but because they sell one time products.
and here's the thing: anyone can sell anything, once..
SaaS, or subscriptions in general, are the antidote to fraud online. because as a vendor you only get < 10% of a customer's lifetime value from the first purchase. to earn the other 90% you have to serve them, which means your solution needs to do what it says on the tin.
therefore, instead of thinking "marketing guru == bad," think "one time huge purchase price == be careful." because the honest marketers i know prefer to charge customers for years, not moments.