Without that information, you cannot draw conclusions about the size of the effect of how they treat out of state students. And if you can't draw conclusions about the size of the confounding effect, you can't trust anything in the chart.

Yes, you have to draw on some common knowledge outside information to know that the size of the effect is not large.

Footnotes aren't there to handhold us all the way from set theory axioms to sociological conclusions. There are a host of potential confounding effects that could be large but common sense tells us probably aren't for various reasons. This is one of those.

This is not something you can hand wave away with "common sense." Common sense would suggest that college-educated young people are moving to Massachusetts (Boston) and New York (NYC), but the article shows the opposite. If the effect is clearly big enough to affect those states, what else does it affect?

E.g. why does Kentucky show in-migration and Kansas out-migration? Why does Alabama show out-migration while Louisiana shows in-migration? Arkansas shows in-migration but Missouri shows out-migration? Oklahoma shows in-migration but Kansas shows out-migration? College-educated people are moving to big cities in Wyoming but out of Nebraska? Are Connecticut and New Jersey showing a net in-migration of college-educated people because Connecticut and New Jersey residents are going to school in New York and moving back home?