Ahh, that's about the time IIRC that Dell started soldering in their wireless chips on the XPS 13. Previously you could swap them out for a different chip with better drivers, now you're stuck.
Embarrassing decision. All in the pursuit of a slightly thinner chassis, I imagine.
My guess is they are using Intel CNVi (https://en.wikipedia.org/wiki/CNVi), which moves some of the WiFi/Bluetooth hardware into the CPU/Chipset. It saves money.
What do you mean by saves money? Couldn't Intel be using shenanigans (i.e. anticompetitive price incentives) to get device makers to use this type of card, so the device makers are then tied to Intel wifi and can't switch without redesigning their motherboard?