I don t think that is true, unfortunately.
Max. resolutions of the m3 are
4096x2304 24Hz over hdmi
3840x2160 60Hz over DP
3840x2160 60Hz over eDP
I5/i7
4096x2304 24Hz
4096x2304 60Hz
4096x2304 60Hz
Both support 3 displays total.
My guess is that there is only one physical connection (DP 1.2) going to the surface connect port while the other two are used for the internal display (eDP) and the DP (DP 1.2) on the surface.
Maybe this could have been realised through an Alpine Ridge controller, but i am not really sure.
I hope i am wrong though.
Next gen Intel CPUs (Cannonlake) should have much better 4k support.