I don't hear anyone in apug saying so but folks in the D forum consider ISO as one factor in exposure. They talked about the exposure triangle. But however, one control the amount of exposure by changing the subject brightness (if that is possible), changing the aperture or changing the shutter speed. The ISO doesn't change the amount of exposure the film gets.
the analog analogy equivalent of the exposure triangle is the exposure waterbucket.the aperture is the size of the valve;time is the filling time and ISOis the size of the bucket. the object is to fill the bucketto the rim. too much waterand the bucket overflows=over exposure;not enough water and the bucket is not full=under exposure;filled to the rim is perfect=perfect exposure;so whatever you change (aperture,shutter speed orISO)just make sure to fill the bucket;anything goes,but with analog,the bucket has a fixed size.