Generally speaking, like "sancta simplicitas", "holy fool" can be used both positively and pejoratively (and both literally and ironically): much depends on the attitude of the speaker.
You might say that even where it's used in a positive sense, there's a little negativity present: the phrase is naturally ambivalent.
In your case, it seems more likely that the speaker intends a positive application, especially if other positive terms such as "vision" appear in the context. But by using the phrase, the speaker to some extent distances himself from Van G: there is an element of concession.
All the best,