He was definitely duped by Benny. Him underestimating was part of the dupe. He did not just come up with this shit on his own. Someone fed it to him, and it's most likely Benny. And he slurped it all up. And yes, he made the decision, this is well documented. You can't just push the blame onto advisors. In that case Hitler would be innocent, he could just shove everything on his mad advisors.