The only thing I can figure is that the guy stealthily unfastened himself, leaned against the door in the steep bank, then slipped the latch and let himself fall out head first. It seems to me that you COULD get out pretty quickly that way - the squeeze is in getting your feet up to the threshold and swinging your legs out onto the step. If you don't care how you land, then it could happen pretty quick. The pilot could have been looking at his instruments and straight ahead - in a steep bank looking down into the bank can mess up your coordinated turn. When I flew for photographers, I always was watching my horizon and my instruments, not whatever they were taking pictures of.
This did bring to mind how my flight instructor used to pop the right hand door open and hang his leg out on a hot summer day. I always teased him about losing his shoe and having to walk back across the flight line with one shoe -- but I never worried about losing my flight instructor . . .
IF you could get the door open far enough at cruise speed to actually exit the aircraft.
I've got about 150 hours in the 150/152 and a similar amount in the 172 and it'd be tough to hold the door open at cruise speed while you wiggle out. The doors on these aircraft hinge forward into the airstream. You'd have to hold the door forward, wide enough for you to fall out, into a 100 mph wind, while you exit the aircraft backward, all without calling attention to your movement? Tight isn't the word for a 152 cockpit. The instructor would have been alerted immediately to the roaring, loud enough to make you think the plane was coming apart. The noise would commence as soon as the door started to open, causing a slight, but noticiable pressure change in addition to the noise. Something isn't quite right with this story.