Actually I'm not sure Nikon truly understands the AF-On button. It is possible they view it like a hood ornament on an expensive car.
The dual thumb buttons were introduced 16 years ago with the F5 and have appeared on every integrated grip Pro camera since. That's at least 5 design evolutions depending on how you count the various h and x models.
In that 16 years I don't think any Nikon engineer ever actually turned one of these cameras into portrait mode and took a shot, using the grip, and using basic AF-ON/Focus lock procedures that we use.
If he ever had, he would have figured out that his shooting strategy is useless because no Nikon grip has ever had the same two buttons. Canon figured it out. See their recent grips.