Kind of yes but also not really. The important thing is that the laws of physics are measured the same in all inertial reference frames. Consequently there is n

Kind of yes but also not really. The important thing is that the laws of physics are measured the same in all inertial reference frames. Consequently there is no non-contradictory sense in which an experiment could distinguish an absolute rest from other inertial motion. It's therefore not possible to talk about motion under constant velocity in an absolute sense, it must always be in relation to something else. Any other body could have been chosen as a (relative) rest frame but the CMB is uniquely convenient to coordinate measurements with due to its age and the extent which it permeates the universe. Physics is still the same in that frame as any other inertial frame.