I don't like MM, and he is certainly not doing the right thing here, but I have to agree with him on the semantics. If he can just leave whenever he wants to he isn't really a prisoner.
I'm not sure to what extent that was really true though - it seems his leaving depended on Happy Medium causing a scene which depended on somehow getting that Ouija board to the guy, IIRC. That's a few too many things that may be out of MM's control to really say he could leave whenever he wanted to.
To further split hairs: what if you have a surefire escape plan that you can pull off whenever you want to, but it takes a great amount of time? I mean, when that time is longer than the remainder of your sentence, then surely you are prisoner. When the escape time is longer than the reaction time needed for what you want to react to, you're effectively a prisoner.
There's also the hints and rumors about MM helping other inmates (like Little Boy Boom) escape, not to mention him getting brought in for questioning whenever an inmate escapes (as discussed in the opening strips of this episode. It's safe to assume that, because of his prior history as a superhero, he's suspected of having insider knowledge of Black Ark's tech, although nothing concrete to justify locking him in solitary.
I strongly suspect he used Happy Medium as a distraction for FF, to break up her date with Clark and leave him undefended. He clearly has some access to the outside (for spying on FF, leaking info to the press, etc), although how repeatable his escape route is is questionable.
While MM has a point... one could ask why he stayed there in the first place. Sure, he (apparently) had some friends there, but learning about Clark was the reason he needed to execute his escape plan? I can't imagine that the place was so much fun that he preferred to stay instead of leaving and doing whatever he had done before and what caused him to get there.
So: Him being here does not mean that he could leave whenever he pleased, it just means that he managed to leave this one time. But it seems to fit perfectly to MM's personality to say such a thing and maybe even believe it himself.
