The paper cited is published by the Linux Foundation, which manages overall development of the Linux kernel, and employs the two lead devs, Linus Torvalds and Greg Kroah-Hartman, directly.

Greg's responsible for much of the device support himself.

I also noted that the claim isn't specifically referenced, but I'm generally inclined to believe it's factual.

Confirmation bias would be seeking out specific information to confirm a belief.

The claim dates back to at least 2008, where Greg KH states it was independently verified by Microsoft:

http://broadcast.oreilly.com/2008/10/how-linux-supports-more...

I've seen quotes from you and I've even heard from you in person that you believe Linux, the kernel, supports more devices than any other operating system ever.

I can back it up by that's true, and it's been independently verified by somebody from Microsoft.

Greg's a developer, not a marketing type given to inflated statistics.

That said, I've asked if anyone's got sources.