Photo agencies can only accept pictures that represent "a moment in time." The picture can be tidied up, but can only be a single image. "That picture" was a composite, or a montage, so didn't represent a single moment in time. This rule applies even to merging images from the same shoot.
Obviously if the picture had been simply posted by them on their Instagram feed, it would have been fine. But it wasn't. It went to the agencies with the intention of syndicating it world wide, so not fine. HTH.