Not that long time ago, browsers could not open PDFs as well. Now all browsers come with PDF reader written in ASM/JS. I see nothing that prevents browsers doing the same for EPUBs. There exist browser extensions that do exactly this already. Its a matter of EPUB format gaining popularity.
My mental analogy is, you can also have offline apps on Android. You can specify this in app manifest. But internet access isn't exposed to the user as a permissions.
Like the author says, Google already injects online fonts into the EPubs they generate. Meanwhile PDF is a battle they've already lost
And also probably why we still have to rely on 3rd party hacky browser extensions to be able to save web pages as a single file.
I didn't notice that before, but now I will actively avoid Google generated EPUB files.
Unfortunately, that feature got killed when Edge became a Blink-based browser. But Edge Canary got a new implementation of that feature a few months ago, so it should land in stable eventually.