This is IBM terminology. “Inverted T” was the name for the arrow key arrangement on ThinkPads (1992) later adopted by the Apple world after the IBM-designed Powerbook 2400c (1997) made their layout the new standard for all subsequent Powerbooks/iBooks/Macbooks.
It wouldn't surprise me if the term dates all the way back to 1984 with the Model M keyboard, but I can't find any reference for that (checked Bitsavers etc) besides You Can Tell By The Way It Is versus the earlier arrow-less Model F keyboard.