The keep bookmarks functionality is indeed only supported by the merger tool. I opted by this because the bookmarks are preserved entirely, even if the target page of any of the bookmarks is not included in the final document. The current code is unable to rearrange the bookmarks tree because the bookmarks action functionality can be complex enough to determine if a specific bookmark item should be kept or not. The merger tool is usually used to merge complete documents, so this is not a problem, but the same doesn't happen when splitting.
If having all the bookmarks in the splitted documents is not a problem, you can also use the merge function to split. But you will not be able to use some of the specific split rules, such as split by file size.
