Says who? It showed up on Motorola documents that Droid Life posted that it's supposed to get Ice Cream Sandwich
Read that post again: that's the author's best guess.
Motorola has neither confirmed nor denied that the D3 will get an official ICS upgrade. So, the real answer is that we do not know yet.
(BTW, I agree that it likely will get an upgrade, though perhaps not what most people are thinking - i.e., it may be missing many of the features that people are associating with ICS. But, that's also a guess.)