Further, wt can get permission to use original art or they may otherwise have the right to use non-original art. Did you forget to read that? The only point is that wt artists have the resources to create original art and logically that is what they do. If they don’t, you wonder what is the reason.
I presume that they would acknowledge the origin of any art-work in the publication. That would be the ethical thing to do.
For me it was almost a lifetime ago but my understanding at the time of its publication was that the artwork in the Revelation Climax book was original i.e. from the Borg’s own graphics department in Brooklyn.