If you don't believe the spec, go ahead and sacrifice an RPi or two by loading them more. You aren't sacrificing much if it smokes. It is probably a little conservative but its like a parachute to those of us who don't sky-dive...not to be counted on but better than going down with a burning aircraft. If you feel like a river-boat gambler, go ahead and burn 'em. I would not design for even that much 'cause I like stuff to last. The CPU will throttle back if it gets too hot but that probably won't help a GPIO overload.
I expect neither Broadcom or the RPi Foundation want a lot of fried cpus out there but the non-profit educational foundation's motives may be purer than the corporate marketing approach you are probably more familiar with. The primary customer is schools and kids and it would be counterproductive to the educational goal to have lots of frustrated kids with overcooked Rpi's.