Based on the description on their site, the controller includes a built-in battery: "8.39 Wh Li-ion battery, 35+ hours of gameplay... "
That was disappointing for me. Specially condidering the Steam Frame's controllers make use of AA batteries: "One replaceable AA battery per controller, 40hr battery life"
AA Batteries might not be as convenient to use, but being able to replace them is a great advantage. All my Xbox360 controllers still work fine, but none of my PS3' Dualshock 3s.
The official docking station could be used to recharge (rechargables) AA batteries so the functionality could remain the same.
I'm alright with this as long as the controller is easy to repair, which Valve has been pretty good about with the Steam Deck.
If swapping batteries is a fast 5-10 minute process I have to do every 5 or so years, and the batteries are widely available and reasonably priced, that's a win in my book.
If they're not a standard battery design, then they will not be widely available or reasonably priced.