Well I hope Microsoft has next to no input on it then

Microsoft develops Windows game mode, so the user-facing bit of the system and of course such a major sponsor surely has general "don't do anything that hurts our brand" clauses in the contract with Asus but other than that there has been not a single piece of evidence that the handheld will be anything but a more high-profile version of https://www.8bitdo.com/ultimate-3-mode-controller-xbox/ which is also just branded 3rd party hardware. IMO there is a decent chance the identical hardware will also launch as a SteamOS version with the buttons then carrying Steam branding.