IMDB notes the oddity, but has no explanation. Does anyone know if this perhaps occurred in another version of the script, or a deleted scene, or know of another explanation?
Good question. I can offer only speculation, and I have never read anything to suggest this was in an alternate version of the script.
It's certainly possible that the artist didn't fully understand what was happening in the movie and was just shooting for something "sufficiently sci fi". Also, the Reliant was certainly "Treky looking" but not as recognizable as the Enterprise, which is one of Science Fictions most well known icons. Back then it was probably crazy not to include the Enterprise on a Star Trek movie poster.
In the movie Dr Marcus (the elder) sends a message to Kirk asking "why are you allowing them to take Genesis?", which leaves Kirk confused. Later Dr Marcus the younger will also pick a fight with Kirk and accuse him of allowing Genesis to be used for military purposes Perhaps one or both of these scenes was part of the preview, and the poster was created as a sort of companion with the preview?
Sometimes comic books use, for lack of a better term, "gimmick covers". They depict events that not only do not take place in the story, but sometimes counter it. I seem to remember a cover for what I think was Ghost Rider shortly after the Nightstalkers stuff. It depicted Morbius - a "good" vampire and member of the Nightstalkers- attacking a human on the cover and had some sort of shocking tag line. Of course Morbius was an ally to Ghost Rider in the comic and it was some other vampire doing the work. I wonder if whatever drove that sort of design was at work here?
Someone deliberately did not want to show the Enterprise and the Reliant battling on the poster, saving that for the movie?
The Reliant was originally a Constitution class ship like the Enterprise, but they redesigned it to differentiate it on screen as they felt it would confuse people during the space battles. I wouldn't be surprised if the poster was designed prior to the change.