I'm not seeing anything indicating that it's limited. You might be referencing the list of universities at the bottom of the page, but my impression is that that's just a "leaderboard" representing which universities have claimed the most domains.

Can you point to/quote the limitations?

I put in my @student.gla.ac.uk email address and it told me this:

"It appears your email is not associated with an eligible university. If you think this is a mistake please email us at [email protected]"