That's true and he also used them in the war against Iran.
There are many possibilities. Syria might have been the destination. Didn't some Iraqi pilots and planes flee there early on? Hell they might be in a hole in the ground for all we know.
There is also this ironic and darkly amusing possibility. When one of Iraq's top scientists was captured, if you'll remember, he claimed that Saddam never supported them with proper funds so when he asked for progress reports they'd basically trump something up to trick him in to believing all was well.
Not sure how much we can beleive of that though. Can't deflect forever.
I wouldn't trust Putin necessarily, but if three of the top intel agencies in the world believe a thing then it's hard to believe its completely baseless.
Bottom line; we went with that justification and found nothing. Not good no matter what one believes.