He's not totally wrong here. The "IMU" you're using consists of a fix, clock, compass, and airspeed indicator (in my case). You're doing the same basic stuff that inertial guidance would do but slower and with less accuracy.

And going back to the great-great-great grandparent comment, I've heard from instructors and other pilots that the 'dead' bit of the word came originally from a shortening of 'deduced'. Something like 'deduced' -> 'ded' -> 'dead' or so. I'm not totally sure of the accuracy of that etymology but it sounds plausible to me.