Where do you see that SteamOS Holo is GPL3? A package is not required to be GPL, and most of SteamOS is a customized arch installation, there's no guarantee that SteamOS itself is GPL. And which repo are you talking about, I don't see it on their gitlab for SteamOS Holo?
As someone else said, the driver is in Steam, not SteamOS. Even on a Steam Deck you have to run Steam in desktop mode to have the buttons on the deck work.
Its been a minute since I've been on desktop mode, but aren't they just a trackpad at that point and none of the button/haptic functionality exists outside of moving the mouse and clicking?