The highly respected researcher William Addams Reitwiesner considered that the idea that Herman was a son of Johann Wilhelm, Duke of Julich Cleve and Berg (1562-1609) was fanciful and unlikely.
See http://www.wargs.com/family/ancestry.html#Note3208
Has anyone any confirmation or better information?
No comment in two and a half years? Here's the Familypedia page for study:
http://familypedia.wikia.com/wiki/Herman_op_den_Graeff_%281585-1642%29
In addition to adding a specific curator note to Hermann, I've also added one to the purported father Johann Wilhelm, count of Jülich & Cleves to hopefully minimize this from re-occurring.