Just drawing on experience (both in college and current, if I'm being honest), but drinking a 12 pack of light beer and drinking so much that you blow a .28 several hours after the fact are entirely different levels of "heavy drinking".
I'm not totally sure of the timeline but it sounds like they were drinking for at least, what, 6-8 hours? PM probably drank closer to two 12 packs worth of whatever booze to get to that level in that time frame.
A cop breathalyzed me once (not because I was driving but because I was underage) after 8 or 9 beers in a much shorter time frame than that and I blew a .07. I still remember that because I was astounded it was that low. I know PM is not a big guy, which obviously makes a difference, but I was at most 160 pounds at the time (wish I could say I weighed that now haha).