A game community can be more than just talking about criticisms all the time. Just like how here some of the biggest threads here in the OF are screenshot threads. I don't have/use Reddit and Twitter because I don't care for catgirl fanart and don't care for the platforms in general, but I won't deny that that's probably where the majority of the players interact with the community and it's not here.
It's also funny that you bring up Discord when you and your buddies have a Discord that is promoted in here for the people who all have a problem with the game and tend to reply back-to-back from each other. I don't think the anti-Zodiark, pro-Mommy or "white knight" factions are energetic enough for a Discord considering anti-how-the-story-went people in any media are much more driven, but even if they theoretically did and went in specifically downvoting ideas they didn't like, you would think there would be other groups doing the same thing but upvoting. And ultimately Reddit is a pure democracy and if more people don't like it, it will end up in the negative or at 0 likes but with hundreds of comments.
Can't speak to how the FFXIV main Discord goes though since I don't use it either.
I'm sure that SE isn't blindly choosing Twitter or Reddit on a whim. They themselves can certainly see how many people are using the forums and made a decision to not use this as a gauge of the audience reaction compared to the others. That said, I don't think Yoshi-P is specifically targeting Reddit users when he puts together ideas for the game, that's conspiracy theory nutter talk. But it can certainly be used as a test how things will be received and to SE, an OF forum user's money is the same as a Reddit user's money.
If anything, we're further down the rabbit hole since everyone here is already paying while non-players could be in the Reddit checking it out. How many times have you yourself said that you were going to quit but you're still here? I remember you making a big deal about leaving in the first Endwalker rant thread in December, but here you are still. Meanwhile I have spent years asking my brother to play the game with me to no avail because he doesn't like MMOs, but he was convinced to play after spending time on Reddit and seeing the reaction to Shadowbringers there. He never bothered to make an account for the official forums, and out of the people I interact with in-game and out, I'm the only person I know who even bothers with the forums and was told it's a cesspit, but all those people still post things from Reddit and Twitter in our group chats.