> If those students/graduates move to other European countries (which they mostly do), they'll be counted in their destination country, evening things out

If they move to the Netherlands, the NL will only count what % of its citizens have degrees, not everyone else (because many may go back or move elsewhere after finishing their studies, you can't count them easily alongside the rest of your population; students are usually counted as some sort of temporary resident, if counted at all). And then there's also the UK which is a top study destination for Eastern Europeans.

> Where did you get the idea most Europeans get a degree

I come from one EU country, live in another, have friends from all over the EU, have visited as a student and now working adult. Most young people pursue degrees, even in forestry or tourism or whatever topic. Of course some will fail getting them, but 30% of under 30s is absurdly low for stats to be reliable.