As
@arn noted, the rumor comes from a source that is relatively new to Apple rumors (Max Weinbach). He may have good sources, but we won't know until his claims are verifiable.
If you watched the session on implementing dark mode on the Mac from WWDC 2018, you'll realize its not as simple as it sounds to implement this. It's much easier on an app basis than it is on a global basis. So it would be no surprise if this takes longer to implement than hoped.
I use Twitter's dark mode all the time on my iPhone and I've started using Outlook.com's implementation on my PC. It definitely can relieve some of the strain of a bright screen. And its invaluable at night. So, I will be happy to use it on my iPhone globally. Hopefully it will come with a sunset - sunrise feature to switch modes automatically, or even an ambient light sensor, much like the Books app does.