I agree about those writers' complexity, Don. I took the critic's meaning in a broad sense, in that they all did often compose conceptually dense writing which was also sublimely simple and direct in expression. This is surely one of the miracles of Dante's poetry--whenever, that is, he's not writing like Arnaut Daniel!
You and others might be interested to see a parallel discussion on this topic that's been going on at an Italian Studies listserv, with me, some other people, and above all our own illustrious and articulate Adam Elgar:
https://www.jiscmail.ac.uk/cgi-bin/w...ian-studies#16.