The sync always downloads/uploads the entire contents of the database, except the media files (which make up the largest part of the database). But Diarium does a merge of both local and remote database if it needs to, like in the situation you describe. I still recommend editing a synced journal, because when you edit the same entry on two devices, the app cannot say for certain which the "correct" entry is. For example, you create an entry, sync it to both devices, then on device A you give it heading "A", on device B you give it the heading "B". Now when they sync happens, there's a conflict between heading "A" or heading "B". If this happens for media files or the main text area, Diarium just keeps both, but this can lead to duplicated text.
Your example is a bit easier and both entries (the one from macOS and the one from iOS) should persist in the journal. You should be able to easily test it out, did you see different results?
