"It boils down to a single “new physics” ingredient in black hole physics as soon as quantum effects are being considered: from now on, when quantised particles and fields are considered, only those general coordinate transformations are permitted that map space and time continuously, and they must be one-to-one,"

ISTM the argument hinges on the idea that conventional co-ordinates for a black hole are one-to-two (i.e. two "normal space" regions in a Penrose diagram) and this is the source of the quantum state paradox/firewall issue.

"the black hole has no interior"