"Lots of websites think “Vertical Screen == Mobile User”!" No, they don't - They think a 720px wide screen is a tablet user, which is probably a fair assumption.
I could equally write "Just because I browse zoomed in 500% doesn't mean I'm on a mobile"
When you zoom in, things get bigger, so fewer of them will fit on the screen... You have a choice between horizontal scrolling, or the website showing you the version of itself designed for smaller viewports.
Looking at the "How it should render" screens - I just checked the Guardian site, and that is exactly how the website looks at 1080px wide. The author admits that because of their zoom solution they are running at an effective width of 720px, so maybe just.. don't do that??
The proposed solution doesn't seem to solve anything, because physical size of the screen isn't the full story, it also depends on how far away from the screen the user is - which the browser can never know.