It might mean that the anime '86' ends in a way that is related to the light novel it is based on. Maybe the story conclusion in the anime is similar to or directly from the ending in the light novel.
Well, '86' is an anime. When it says 'end in light novel', it could imply that the final arc or the overall conclusion of the anime's plot is sourced from the light novel. For example, the way characters' fates are resolved in the anime might be exactly how it is in the light novel, which serves as the original source material in this case.