It's just assuming that π is 5 in this specific scenario, just like it's reasonable to assume the existence of a spherical cow in a frictionless environment. Yeah, if you use the results of this problem to develop a real cylinder you're going to have a bad time but nobody is doing that all what's the problem?
Nobody is saying that from this point in time and going forward π = 5 and now math is broken forever. People need to chill