I can already see the series of events that would take place if Dark Mode was implemented in iOS 12
- Users find themselves toggling Dark Mode on and off when going outside or in a room that is brightly lit because it is difficult to read the screen.
- Users ask Apple to add a control center toggle for Dark Mode because it's incovenient to go to settings > display every time.
- Apple complies by adding a control center toggle.
- Users then complain about how it's inconvenient to always have to go the control center to toggle it and ask for an automated process.
- Apple complies and uses the ambient light sensor to detect ambient light and automatically toggle Dark Mode on and off based on ambient light.
- Users complain that they like how "cool" dark mode looks and want to leave it on even when it's not dark so they disable the "auto detect" feature and....we're back to "it's not convenient to toggle it on and off".
To be fair, that could really be said about many features, so it's kind of neither here nor there in that aspect of it all. (And realistically speaking dark mode might actually work out better in bright environments too.)
Ultimately having the option doesn't seem like a stretch at all, especially given that they now actually did it for macOS.
That said, seems like this thread isn't really about it as it was just a misunderstanding of a new type of notifications and how they are presented, without really having much to do with anything like dark mode.