Earlier quoted context omitted.
The OP is talking about things like media queries that only look at screen width and imply other things like input method from that. IIRC SquareSpace does exactly what they're saying: on desktop sites will have a horizontal list of links but on mobile it's a hamburger icon that takes over the entire viewport when clicked. Functionality exists to target pointer type and things like that in media queries, it's just ver…
But that's what you want , no? I just visited SquareSpace and their homepage seems to work as I'd want. When my viewport is narrow enough that there isn't space for the horizontal navbar at the top, it switches to hamburger menu. Surely this is better than cutting off the navbar and half the page content in the middle vertically, and requiring the user to scroll horizontally?
The point is that the viewport-covering menu is huge because it’s designed to accommodate touch (i.e. imprecise) interaction. It could be much smaller when the user is using a mouse. But you rarely see such accommodation being made. The assumption is low width = mobile = touch.