For the sake of this discussion, I'm going with the nationalistic vibe of the order: anybody who isn't a citizen of the USA (presumably to limit risk of AI-supported action against the US?).
But that in itself is telling in a way: if national security was a true concern, access should be limited to people who passed background checks.
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.