My best (frankly uneducated) guess would be that they trained the model with famous movie posters as the objective, and still frames of the movie as the training data. Then they gave frames of these movies to the ai to get a poster out.

I don't think they look as if movie posters were the training objective. No texts, no large faces of leading actors, wrong aspect ratio, unusual colour palette.

Oh, so image data? I got the impression they used a text-only description as input.