The AI does cheat with some things (naval attrition, for example), but manpower is not one of them. I assume you're talking about their manpower levels exceeding their maximum manpower? That can be caused quite easily by having the max manpower drop after already reaching it. I'm betting that the AI had 180k MP and you just took a couple of provinces from them, making the max manpower drop. Wait a few months for them to reinforce some armies and you'll see that their MP level won't go back above their maximum.
I can guarantee, having played many, many hours of this game, that the AI does not use a manpower cheat (unless you're using a mod that makes it do so). In fact, draining an AI country of its manpower is one of the best ways to defeat it.