why does × mean "close" when it's in the upper-right corner of a screen

But even ignoring the counterpoint to the idea that a UI element must be contextual within some category to be understandable, the idea that the hamburger menu isn't a standard OS abstraction is also just silly. Hamburger menus are for mobile devices, and in your points you're comparing them with navigation used on operating systems meant for desktops. On Android — the most-used mobile operating system in the world — the hamburger menu icon is part of the standard toolkit, is built directly into their SDK, and is called out explicitly in their specs for Material Design.