Taken from there, it implicates that the error will be greater when you're flying up higher; is that the correct assumption?
Yes, I guess so.
Difference is 4ft per thousand for every 1*C difference from ISA - that is to say that if at 1000 ft in ISA-10 , error is 40ft, whereas at 10,000ft in ISA-10, error is 400ft.