Good point. The unboxing video isn’t exactly clear about that, though it says, “This video doorbell can be powered two ways….” The Installation Guide in the Help Center says to install the battery even in the “Wired Installation” section.
Again, I don’t know why someone wouldn’t want to install the battery for backup power, and maybe that’s the thinking behind the Help Center article, but it’d be good to have a definitive answer for other use cases.