Quick clarification: I'm pretty sure there's no uploading at all; it uses local storage for the filtering too, which is important because uploading a ROM tends to be risky legally
Absolutely, I should have made it more explicit, thanks for clarifying it. That's what makes this possible and legally compliant.
I was pretty sure you understood based on how you described it! This stuff is very easy for people less technical to get confused about though (which is not at all their fault!), and from what I've seen in the subreddit where I found out about Hackdex, it seems like there was a lot of confusion about it early on, so I figure it's better to be on the safe side.