Umm... Button was on the inside, and jinked his wheel to the right TWICE as he was sliding.
DC likely trusted that button wasn't going to try to enter the turn perpendicular to DC's (proper) line.
Having not driven an F1 car myself, I assume that DC saw he was too far back, and _turned_ into his corner, as Button _slid_ into the apex. It's probably also hard to see in your mirrors while looking towards the apex.
I recorded the race on my PVR if you want a copy to see it again.