That's basically the inference I've taken all these years too, Moiraine. I remember learning about "binge drinking" in Home Ec in middle school, and the situation they dramatized involved someone having between 15-20 drinks in a night, to the point they passed out.
I would assume two things about this 'five drinks every two hours' rule - first, the creators of it are probably suggesting if you extrapolate that over one night it could end up being 10-15 drinks in 4-6 hours. I'd considered that binge drinking. Second, I bet a lot of people view binge drinking as some continual problem that happens after repeated (and copious) drinking. This rule is sort of saying hold up... just because you haven't been drinking a ton over the whole night doesn't mean you can't go overboard in a couple hours.