I wouldn't so much say that it's him lying and more so just committing a rather petty logical fallacy. Don't support the deal? Then you must be a warmonger. It's the strawman fallacy, one we see all too often in news media. It stifles perfectly necessary debate, which are important for highlighting concerns and refining ideas, as it's a silence tactic. Making those who oppose you seem so bad that no one will want to listen to them. I say the President may not be lying because logical fallacies aren't inherently intentional, he could genuinely believe it, we just don't know. But seeing as he is politician, and as such a learned man, I would imagine that he's fully aware of the fallacy, as unfortunate as that may be.
The Iranians refuse to give up their nuclear program. It's a ten-year hold on their nuke plot or the United States goes to war against Iran (which may very well happen anyway after the agreement expires). Therefore, this assessment is probably accurate.