Nah it didn't look like a roided physique but who knows. It's not outta the question though so I won't flame you ... 25 pounds of muscle (didn't seem like he put on much fat, actually looked more cut) in 5 months is an insane amout to put on, especially for someone already at 240.
People that big have a really hard time putting on and maintaining so much muscle. It's either an insane amount of lifting, cardio AND eating, or the fishy route. I remember Gonzaga tried really hard to bulk up after the Werdum fight, and he barely managed to put on 5 lbs of muscle and as much fat... and Gonzaga seems to have a naturally bigger frame and gorilla muscle tendency compared to Mir.