The line of thinking is that there is a difference between semantics (actual aboutness) and syntax (mere structure). The classic example is watching a colony of ants crawl in the sand, and noticing that their trails have created an image that resembles Winston Churchill. Have the ants actually drawn Winston Churchill? The intuition for externalists is no. A more illustrative example is a concussed non-Japanese person muttering syllables that are identical to an actual, grammatically correct and appropriate Japanese sentence. Has the person actually spoken Japanese? The intuition for externalists is that they have not.
Not everyone is in agreement about this, although surveys have shown that most people agree with the externalist point of view, that meaningfulness does not just come from the head of the observer — the speaker creates meaning since meaning comes from aboutness (semantics).
The most famous argument for semantic externalism was put forward by Hillary Putnam, I think, in the 60s. Roughly, on a hypothetical Twin Earth which was qualitatively identical to Earth, except which water was not composed of H2O but some other substance XYZ, an earthlings visit to Twin Earth and looking at a pool of what appears to be qualitatively identical to water on earth and stating “That’s water” is false, since the meaning of water (in our language) is H2O, not XYZ. To externalists, the meaning of water = H2O is a truth even before we’ve discovered that water = H2O.
I think the argument for AI art being pseudoart follows a similar line of thinking. Even though the AI produces, say, qualitatively indistinguishable text from what would be composed by a great novelist, the artwork itself is still meaningless since meaning is “about” things. The AI, lacking embodiment, and actual contact with the objects in its writing, or involvement in the linguistic or cultural community that has named certain iconography, could never make (externally) truly meaningful statements, and thus “meaningful” art, even if (internally) one is moved by it.
If one is to maintain the internalist position, that any entity that creates aesthetic mental states qualifies as art, then it seems trivial, since literally anyone can find anything aesthetic. Externalist intuition effectively raises the stakes for what we consider art, not necessarily as a privileged status available only to human creations, but by arguing that meaning, and perhaps beauty, does not only exist when we experience it.