That is not entirely accurate, the DP signal can be embedded in a TB stream (like on the Apple TB Display, where the actual signal in the TB cable is TB data) but a TB device can demux the DP data and output DP data in the DP/TB connector. The DP data actually uses pars of the total available bandwidth with Thunderbolt 2 since there is no dedicated 10 Gbit channel for video data as with classic Thunderbolt.