Well, you seem to be coming from this from a perspective of what a webpage tells you about WinPE.
And you perhaps think that WinPE is only the thing that Windows installation uses.
But it's more than that.
You can use WinPE without even knowing that it's part of the Windows installation. I used WinPE before I knew it was somehow part of how windows installs itself.
I remember once somebody's computer went down and I said here use this computer and I let them use a computer that booted off WinPE. They could browse the Internet and run some basic programs.
Infact, if people had issues with, say, deleting a file in windows, and then they could boot off a linux USB/CD and do it from there. Or, sometimes they could alternatively do the Windows equivalent of that and boot off WinPE. Before I knew of WinPE I recall there being a BartPE which was a basic cut down Windows OS which
years later I found out was based somehow off WinPE.
Really it's hard to see how somebody can't know that WinPE is an OS. WinPE isn't so well advertised though..
Last time I made a WinPE USB, was I ran a program called Macrium Reflect which makes an image(nowadays one might do that from Windows), and Macrium Reflect has an option to make a bootable rescue WinPE USB.
From there I think you'd manually open macrium reflect.. but could do other things from it too.
It's very obviously a lightweight OS!
Infact, in the uses that i've mentioned, it doesn't pretend to be anything else.
We used to use Boot Disks(Dos boot disks, or Win9X boot disks, which are DOS boot disks), then boot disks converted to USB/CD/DVD.. But then people started using lightweight Linux boots or BartPE or WinPE.. to do fixes without loading windows.. Those are all OSs.
Also you have to look at what is an operating system.. well, it loads pretty early on and stays running, and it's a piece of software that runs other software. That's a far from exhaustive definition but WinPE does fit that. And given the examples i've mentioned it's really impossible to not see it as an OS in those examples. If you only know of it as something that windows installation uses then I can see how you wouldn't know it as an OS.