https://www.cnet.com/news/steam-link-app-bluetooth-le-controller-support/

Valve released the Steam Controller back in 2015, and while some people chose to use the controller to play their Steam games on PC (others may prefer their keyboard), it apparently had one dormant feature all this time: Bluetooth support.

Years later, Valve is now enabling Bluetooth LE support in its controllers, which means it had the hardware for it all along. (Normally, you pair the controller to a wireless USB dongle.)