Arbitrary and random are basically the same in this context. The person you're replying to is pointing out they are specific keys for specific reasons because those specific keys can be used to figure out which kind of keyboard it is, they are not random or arbitrary, they are chosen with a specific reason in mind. If they were really chosen arbitrarily, they would not be able to accomplish the goal.

My impression is that Apple's choice of keys was constrained.

The American Heritage® Dictionary of the English Language, 5th Edition defines arbitrary as

> Determined by chance, whim, or impulse, and not by necessity, reason, or principle.

But the choices seem to be based on reason or principle.

they are not random requests. they are keys known to have different locations depending on the locale. by seeing what the values sent when the requested key is pressed, they know how to map the inputs from the keyboard.

that definitely does not sound arbitrary to me.