Right - it doesn't hold up to scrutiny. For one, "not a citizen" is a pretty hard bar to assess online. For another, "citizen" isn't very meaningful here. Many national security incidents have featured a citizen at the core - and it's a really fuzzy indicator of "potentially hostile" and especially "for what reason".

I guess I'm possibly giving them too much credit, but if the people who sent the letter have their head screwed on straight, "protecting national security by disallowing specifically non-citizens from using it" can really only be read as a smokescreen, or at very best a small part of the actual picture.