Unfortunately nearly every graph on that page is intentionally misleading. If you actually adjust the graphs for inflation (where it’s relevant), 1971 looks like just another year.

Lying with statistics!

I get that Ukraine won’t consider the possibility of ceding any territory, nor should they. They probably don’t like their allies even mentioning it.

But, there’s the separate issue of not being able to join NATO with ongoing territorial disputes. Without much context to go on, I would almost interpret this as something more along the lines of “Ukraine could join NATO tomorrow if the dispute went away (by whatever method)”.