I don't buy that argument completely, nothing would be stoping them from just not bundling the drivers with Steam and also not requiring that I launch games through steam (if that is true).

While we could argue about the state of Windows, Steam also did not have to engineer it this way and the requirement of launching through Steam feels deliberate.

From what I can find, as others have mentioned, the 8BitDo controllers don't require Steam to be running to work. I presume the PS5 controller likely also does not (I will test this later)

It's a specific Windows issue too, and not unique to the steam controller. Xinput doesn't work with generic USB controllers, because you know, Microsoft. Hence why you need cope software for Sony controllers.

Valve does deserve criticism for the royal pain certain things are though. For example non-technical users will absolutely struggle to get Proton working without Steam, the process in doing so is purposefully kept undocumented and esoteric. There's 100% a little bit of undesirable obfuscation Valve does to push you towards just using Steam to run their OSS. It's definitely non-Free in the purist sense.