To say that dark mode isn't possible just because 3rd party apps exist with their own design is taking it too far. Yes, it does mean there can't be an immediate, universal, all encompassing dark mode. But there are still many key system features and many stock apps, regularly used by many iOS users, where a dark mode would have some immediate impact. And it would still be open for 3rd party apps to join in slowly.
Notifications, widgets, notification centre, control centre, popup windows, stock keyboard, spotlight. all the stock apps. don't forget many people's core iOS experience is stock apps: Phone, Messages, Mail, Safari, .. the list goes on but all stock apps!
The immediate impact would be pretty wide. Then apps could, over a beta period, start to take advantage of the dark mode. Apple would make apps aware of this setting and able to adjust their UI when the setting changes. Not all apps would take advantage of it, but that doesn't mean dark mode isn't possible!