I found a reference design here (see the image below). I’m guessing the antenna is that trace around C5 and C6. This is the same design in the datasheet.
I also found a document on antenna tuning with lots of equations and the need for some expensive equipment.
Adafruit also has a design with a completely different antenna.
I may just copy the Nordic’s design, spin a PCB, and give it a try. But I’m not really sure how I would tests it’s efficiency.