That just means it shouldn’t be a native app or a web app, but instead should be a plain ol’ webpage that doesn’t try to do app-y things in the first place. The notion that web pages have any legitimate reason to know your viewport size (let alone anything at all about the screen hardware itself) is like one of those “statements dreamed up by the utterly deranged” memes, except not satirical.
Seriously: literally the entire defining principle of HTML (well, aside from the concept of “hyperlinks”) is that the client has the freedom to decide how the page should be rendered, but misguided – or megalomaniacal – graphic designers webmasters front-end web “devs” have been trying to break it ever since.