You're forgetting a couple of things. One, Mother Boxes have been on Earth for a while. While we only more recently got an idea of for how long, we DO see in the film, via the footage and info he has on the Meta-humans, that Lex would have known about at least ONE of them for a while.
Knowing Lex, there's no way that he wouldn't have a hell of a lot more research on these people, and the things he's finding, than just the security footage clips.
So, there are definitely elements of the film that suggest he COULD be being influenced, or nudged, in certain directions. Same as we can infer that Bruce is being influenced from the Knightmare vision. While it could just be a glimpse of the future the Flash was coming from, it could also have been something from, or manipulated by, Darkseid and friends. There's plenty of signs of him in that scene, so why would we ignore it in our interpretations?
Long story short, saying "there's nothing to suggest that Lex was under the influence of Darkseid or anyone during the film" is patently untrue.