I agree with your point and understand how the current text may be
misleading, so we shall make it clearer in the NEP (as done in
https://github.com/numpy/numpy/pull/14529) that both are valid ways:

* Have a genuine implementation of __array__ (like Dask, as pointed
out by Stephan); or
* Raise an exception (as CuPy does).

Thanks for opening the PR, I will comment there as well.
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