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I doubt the author meant for this trick to be actually used, they were just showing how pointers to arrays are typed correctly in a clever way.

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Indeed, one could hope that is the case! :-)

But my point with suggesting the macro applies equally to the more traditional sizeof division. I have seen code that divides the two sizeofs every time an array length is needed. I think it's better to put that calculation in a macro so you only do it in one place.

You are dividing one constant by another -- surely that would be handled at compile time?

You are correct, the compiled code will be the same whether you use a macro or not. In fact, this is true for any C macro. A macro is merely a source code text substitution done by the preprocessor. Using a macro is exactly the same as writing out the equivalent macro expansion everywhere you use it.

My suggestion to use a macro is not because of any difference in the compiled code, but to improve the readability of the source code.

While it may be optimized, I think the suggestion is that instead of using a hack repeatedly, it is arguably better to be DRY and abstract it away.

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