>Another critical optimization boiled down to realizing that the response packet allows up to six keycodes to be reported at once. This might have seemed like a straightforward 6x speed gain, but not so: on MacOS, the keystrokes were dequeued not in the order they appeared in the packet, but from the numerically lowest scancode to the highest. This mind-boggling quirk [...]

Reporting multiple keys down in the same packet is meant to be used for when the user actually has those keys down simultaneously, so it's not unusual that MacOS decided to act on them in order of scan code, because the expected effect would be the same.