If you think of the word "commentary" by itself, it is to commentate something. Hashtags are like tags, they are just there because there is no better place for them as sites like X don't have a separate field for searchable tags like Pixiv has for instance. The artist does not talk about the image when they layout the hashtags there. Symbols such as in symbol-only commentary do commentate, they are like a single word commentaries in a way (or more given the amount of symbols).
Now if we do think as hashtags "artist is commentating something about the picture", it still goes against the commentary tag in a way as that tag is meant for "understandable by the average English speaker" and if the hashtags are in e.g. Japanese, the user reading that commentary is not getting any new information assuming their Japanese level is at 0. If we make this implementation, then all commentaries that solely read "初音ミク" would be commentary request and "#初音ミク" would be commentary.