Yes, that piece with the 2 slots is a threaded nut that can be unscrewed to remove the antenna.
There is a special tool available for these -->
https://gmclassics.com/part/antenna-...hes-on-bottom/
It's basically just a piece of aluminum pipe with some roll pins (1 to grab a notch in the antenna nut and 2 as handles to turn the tool). Not all that difficult to make, but the ID of the pipe does have to be a little bit larger than the antenna nut so it can be slid down over it without having to back out the roll pin.
Some soft jaw pliers would probably work too if the nut isn't on there super tight.