In this case the pin going low has never occured since the processor has come out of reset, so the state of the pin is not ambiguous.
It'd be nice if there were a standard "clearInterrupt()" function, or maybe another argument to attachInterrupt, so that you could do this without having to delve into the guts of the chip's GPIO configuration. (but the ISR should STILL check...)
Thats the point really, and if that were so, then the examples given in the reference would more closely match the reality.