> How does that work? Do they just let foreigners into their University?

Yep, looks that way: http://www.ryongnamsan.edu.kp/univ/en/education/internationa... (although I note that 'the US' is not included in the list of past students' nationalities).

There is also a private university in Pyongyang funded by foreign Evangelicals, including from the US: https://en.wikipedia.org/wiki/Pyongyang_University_of_Scienc...