Good catch. The more accurate word would be "region", which also reflects the terminology underlying the graphics primitives subsystem.
If they wanted to be really precise they for say "display region plus slop region", the latter part being the forgiveness area a few pixels beyond the actual displayed object, commonly used in hit testing with the pointer tool for the convenience of user interaction.
But that's probably a mouthful of trivia uninteresting to most readers. I suspect just replacing"rectangle" with "region" would suffice.
Fun trivia for Mac advocates:
Submenus are physically more difficult to use than primary menus, requiring the user to slow down and change mouse direction from downward to horizontal, being careful to remain close to both the primary and submenu, before resuming downward motion. This relative clumsiness is why the Mac HIG and others recommend keeping submenus to a minimum, avoiding them entirely wherever practical.
But have you ever felt that accessing submenus felt even more onerous on Windows?
That's because they are. By design.
And it's all about the slop region:
From the earliest days, the authors of the Mac Menu Manager were very sensitive to the inconvenience inherent in trying to access submenus, so they built in a generous slop region at the juncture where the submenu is drawn adjacent to the primary menu. It's an expanding trapezoid almost extending the height of the first menu item. This allows us to be several pixels off as we change navigation direction, giving the transition about an easy a feel as it could have.
The Windows design team apparently felt different. If you pay close attention to each, you'll see that the Windows slop region for accessing submenus is much smaller, far close to a rectangle then an expanding trapezoid, requiring slowing down much more and moving the mouse with greater precision.
Whether that choice stems from a desire to be more precise, or just weaker user testing methodology, only those who were in the room at the time it was decided can say.
All we can know is that accessing submenus on Mac not only feels less uncomfortable, but the underlying reason can be measured.
Of course all this became far less of an issue when all the OSes standardized on persistent drop-down, where we no longer need to keep to mouse button pressed to keep the menus rendered.
But if you're a Mac snob, you often have good reason to be. There are many details like this that distinguish the two platforms.