It's not a publicity stunt for three reasons, the first being that their album's pre-order sales is already sky high a week ago according to amazon and the second being that the festival would have to be in on it to cut Green Day's set short like that and allow the camera to turn to the monitor that said 1 minute left, I don't think it's cool for a festival to be known to change plans or set duration on a band while they're playing. The third and last thing that makes the publicity stunt unlikely is that Armtstrong has checked into rehab today and they had to cancel a bunch of tour dates, this is going a bit too far for a publicity stunt IMO.
Another thing that won't count cause I can't prove it is how unlike Green Day a publicity stunt in that form is, I have been a fan of this band since 1998 and they have never been nothing but honest and straight forward with their fans and this is highly uncharacteristic of them.
I know the general vibe here is to disrespect bands who make a lot of money from making simple music but even with the prog snobbery of that vibe Green Day have always been an exception cause they have kept some standard to work they put into their music and lyrics. Among their genre of punk rockers and pop rock; they are the most decent, creative and original.
Now a word against them would be that over the last 3 years Armstrong's drinking problems have been getting out of hand and I'm not sure if "checked into rehab for substance abuse" mean drugs or alcohol but either way the guy really has a problem currently and the way he handled this situation on stage was completely uncalled for and childish.
I do not understand how making short songs is wrong so I can't really comment on that part