Taken from there, it implicates that the error will be greater when you're flying up higher; is that the correct assumption?
What matters is the height difference between you and the station at which the QNH was measured. So at ISA -20 degC, 1000 ft above a 7000 ft airport, using that airport's altimeter setting will cause the altimeter to overread by just 70 ft, not 500+ ft.