Why do you doubt it? You can already run two 2560x1600 displays as well as the internal display at an effective 3840x2400, which is a shedload more pixels than this...
Because of the following:
1) The current TB/DP supports 10Gbps, which 2560x1600 displays does not saturate via each TB/DP port.
2) Internal Display is internally connected to the GPU, so the external protocol doesn't matter
3) The 5120x2880 requires ~15Gbps of data throughput, something current TB cannot do. Thus, TB2 is required with its 20Gbps channels.
This does not mean a new Retina display is coming... There are plenty of screens that already have a 5120 x 2880 resolution already.
Such as?