I think there's people who do check out Product Hunt without posting themselves, although it's probably not a whole lot. I regularly check it out to find new products, or for general inspiration, but that said I do post in there myself.
It's worth it from experience if you manage to get in the top 5 or so, and get featured in the newsletter as well. I've personally gotten a decent amount of users and traffic by posting in there. Plus it can also help to get all sorts of opportunities, after a launch I tend to have people and companies reach out who are also in the space, or looking to partner - it's generally pretty good for networking. But it also depends on the kind of product you're launching, I've had more success with productivity tools (a task management app, screen recorder, design tool...) than with other types of products (directories, communities, niche tools...), you have to know the audience.
Definitely do agree though that it can be a bit of an echo chamber. I wish there were more platforms that were used by "lurkers" where you could post your product and reach your audience. Unfortunately many platforms discourage self-promotion, it's a shame really. I love discovering new things all the time, but it seems nowadays the only products that reach you are the popular ones, it's incredibly hard to discover niche ones. Not just for websites and apps, but it also goes for movies, books, and all sorts of other content.