Terrible though it was, the measurable impact of 9/11 on the airline industry economics were little in evidence just nine months later - not so for Covid. It’s a strange date to use as a baseline for comparisons, that’s all. Why not 20 years ago instead of 22?!
Glasgow’s real issue is that it had some very chunky traffic levels on domestic routes which have fallen away. Historic IT traffic (where Edinburgh has rarely done well) has held up ok and will be a growth area this year. Edinburgh has seen a similar drop on domestic routes but has powered forward on European connectivity and transatlantic traffic, based largely on inbound traffic mix in both cases. Glasgow isn’t really equipped with the same pull in either, so gets the downside without any of the upside.