That flag is to control how dark mode behaves when you force dark mode for a page. It's not for enabling dark mode on its own.
This is not the behaviour observed in Chrome (also Edge, I just tested it) for the #enable-force-dark option.
Basically in Chrome or Edge, #enable-force-dark turns ALL WEBPAGES into dark mode regardless of whether they have a dark theme or not.
Have you ever refreshed a website using dark mode and seen short-lived white flashes during when it loads up? That's the browser's default white background showing up while the rest of the web content is loading. And when I use the #enable-force-dark option in Chrome/Edge, that background is flipped to black and I don't get epilepsy when I visit webpages.
