Saying your game is for players 17+ only and having the game's Discord be age-gated are two entirely different things. A lot of people do not want minors in their Discords for a lot of perfectly good reasons. Moderating a server becomes way more of a responsibility when minors are involved and most people i know who have a server just don't want that put on them so they make a "no minors" rule even if posting NSFW content is also against the rules.