I think you're missing the point. The author wants to know the location of the binary of the shell he's currently running. Changing PATH doesn't help answering

I think you're missing the point. The author wants to know the location of the binary of the shell he's currently running. Changing PATH doesn't help answering this question: it only changes what `which` displays, but the point of the author is that `which` is not the correct tool to help answering this question.