[Quoth]

AFAIK it isn't BT
Some MS Pens also have BT as well and an extra button that uses BT for OneNote or other things (some a second battery for BT). An MS surface 3 or later compatible 3rd party pen works interchangeably on at least 5 Kobos, as does the Kobo original Pen.
Tested: several Sage models, Elipsa and a Libra 2, which while not having notebooks, the Sketch works with pen and buttons. Neither kind of Pen works on Sketch on the Original Libra.

The original was NTrig and MS liked it so much they bought them and swapped to it on the Surface 3. Earlier they used Wacom. MS first added pen support in 1992 on Win 3.1. Win9x had a version and there was an XP Tablet edition. All non-MS HW and mostly Wacom.