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. T…
So IMO it just comes down to some sites doing a bad job of picking responsive breakpoints.
Edit: Out of curiosity I checked MDN, and their media query tutorial has you set the mobile breakpoint at 40em. Default font size at least on desktop is 16px (again, virtual pixels which scale with display-scaling and device size), which comes out to a breakpoint width of 640px: https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layou...