You’re being needlessly narrow minded. Any improvement on that integration that doesn’t use an absolute source of position is still dead reckoning. Do you think ancient sailors used IMUs?
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.
The term "dead reckoning" was not originally used to abbreviate "deduced reckoning," nor is it a misspelling of the term "ded reckoning."
Am I an old man now? Where is my cane? Get off my lawn!