In the article it says the bike they tested was a pre-production model; I would guess that the exposed cable running underneath the top tube will be rerouted internally once the bike hits production. At least I would hope so.

Edited: Although looking at the junction between the seatstays/toptube/seat tube, it doesn't look like there's really a clean exit point for an internally-routed rear brake cable without having to bend the cable a considerable amount. Maybe it will stay external...