They couldn't use the same panels for a thunderbolt display. Thunderbolt 2 has a max data rate of 20Gb/s, but the 5k monitor they announced has a data rate of

5120 x 2880 x (24 bits per pixel) x (60fps) = 21Gb/s

Just over the limit for Thunderbolt 2 unless they want to do 4:2:2 or some other compression technique.