Sites are not detecting a portrait screen and deciding that they're displaying on a phone. Someone who switches their monitor to portrait, or resizes a window to be taller than it is wide, is still going to get the desktop view.
In this case the author is scaling their resolution, and so their browser is presenting itself to sites as 720px wide. Sites see that and show a layout optimized for a screen of that width. Those sites are acting correctly.
The author's proposed solution is for sites to use DPI instead, but this would thwart the preferences of users who use scaling because they want the whole site to look bigger.
In this case the author is scaling their resolution, and so their browser is presenting itself to sites as 720px wide. Sites see that and show a layout optimized for a screen of that width. Those sites are acting correctly.
The author's proposed solution is for sites to use DPI instead, but this would thwart the preferences of users who use scaling because they want the whole site to look bigger.