i would like to bring to your attention that the Hololens documentation is not up to date anymore.
It seems there have been changes and regarding Hololens that are not documented.
There is no way to associate the Hololens camera with the ARCamera anymore as suggested here
What does this mean, that the ARCamera is the main camera now?
In unity playmode the camera image is flipped (2017.3.1f1-p2).
You can not use the unity Holographic emulation window anymore using the xbox controller to navigate for testing in playmode, because vuforia takes over the ARCamera and there is no attached Hololens cam anymore.
The Vuforia Hololens sample from the asset store does not work in 2017.3.1f1 not even in the editor, it throws errors.