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The example is not wrong in and of itself, but somewhat misleading (it was at least for me).The :id should actually be the key of the ref, which is what is being changed by the threads. I think the text tries to allude to the significance of the :id a bit in the final note, in that you will see way more clashes of id's if you run this example with commute instead of alter, but it is a bit obscure.
Please take a look at for the details of how I run it in a multithreaded setting.