I think that it was summed up well by the squidschool video highlighting this problem. There are many, many scattered hubs where people gather for different purposes, and I figure many have their own circle of activity. The boards would be competing with discord, twitter, and other channels people use the communicate.
Though I think it hits the nail right on the head with the boards being in a bit of a catch 22. People come here, see no activity, leave and no new activity gets generated making the next person go: "Oh, no activity" and repeat that cycle. It also might be that forum boards appeal to a certain kind of audience. I imagine someone used to the constant stream of discord content might find a forum unintuitive at first, while (insterting personal experience here) on the flipside someone may find a forum way less busy than a discord, if only by virtue of topics being split up instead of being in a great big pile, so to say.
Maybe taking a focus to more competative splatoon at first would be good. Discord is fine and all, but having a more archiveable way of view discussion could help people understand why something would be impactful. People may know that running ink saver main on stamper is now kinda necessary, but what would be the best ISM amount and why is something that you can't natively understand.
And while I am sure there are video resources on youtube and discord servers with threads that contain the answers, neither can present the math as well as a massive text post can.
Threads like these are amazing to learn exactly why X mechanic works for Y weapon or in general, but turning that into a video/discord formatted thread would be not the engaging content you'd want on those platforms. And while you could put it up as a google doc or something, it takes it away from a great hub of information it could exist in.
Long story short, I think seeing if we can all pitch in to create content and focussing that content around the audience we would want to find would be the way to start getting the ball rolling. After that, hopefully, it'll gather momentum and a bigger audience will flock to the site.