My guess is that he copied Obito's jutsu
Sharingan can't copy kekkei genkai techniques even if the caster use the same thing(otherwise Kakashi could copy amaterasu,tsukoyomi etc.)
I believe the ability to adsorb your self is a different aspect of kamui that is granted in both eyes.
If you think about it,Obito would have to touch himself (I know it sounds weird,don't get me wrong) to adsorb his whole body or part of his body.
But he doesn't do it.Meaning that he only need to touch an external target to kamui it and not himself.
Same goes for Kakashi.He has to concentrate his sight on the target,but when it comes to him he can absorb his whole body without having to look at it.
The eye is connected with the optical nerves,which are connected to the whole nervous system,so therefore with the whole body,so kamui can automatic be used on the body it is connected without any other requirements.
Something like that....