That’s clearly a possibility. I’m not too convinced of that, though.
Manafort was arrested, his records were seized, he was investigated pretty thoroughly by most accounts, he spent several months in jail pre-trial for contacting witnesses before trial and getting caught, he was convicted of eight crimes, he’s now spent even more time in jail and is reportedly not handling it very well mentally or physically, tens of millions of dollars of his assets have been seized and are up for sale this week, he makes a deal with Mueller to cooperate, he knows Mueller is watching him and double-checking him, he knows the federal judge will crush him if he breaches his cooperation deal, he knows he could be brought up on even more charges if he gets caught breaching the deal, a presidential pardon is not a certainty, and yet . . . he breaches the deal.
There is something bigger going on here. My speculation is that Manafort, knowing that Mueller has not yet found the Mother Lode, is doing his part to misdirect the investigation and keep it away from the correct path. Mueller has spotted the tip of the iceberg, but hasn’t learned how deep down it goes. (mixed metaphors, i know, but hopefully you get the idea).
On the other hand, maybe the OP is right and maybe Manafort is an idiot. But i don’t think he got where he was, in his particular business, by being an idiot.