Sure. But most good passive income ideas are good because of how easy they are, and because they're the perfect size for a small market. Usually even one more competitor entering the market is enough to significantly disrupt them. If you are sitting on one of those, why would you post about it in an HN thread that will be archived and trawled by capable competitors specifically looking for ideas? If it only takes one competitor to disrupt your golden goose, why would you post about it here?
While there are many interesting ideas in this thread, I expect there are a fair amount of people lurking with much more profitable and higher leverage products. So maybe just take the thread with a grain of salt and don't think of it representative of a ceiling on what's possible to build in 2019.
Most people are basically lazy and will talk a good game, but not actually do anything.