I think you're misreading it.
This is about reporting holes, but nobody wanted to guarantee that such a thing as a hole exists. So the semantics are: if SEEK_HOLE reports a hole, the content there consists of NUL bytes. That's it (though naturally enough any sane kernel is only going to report large blocks of NUL bytes, for example by reporting the actual holes, and userspace programs are likely to rely on that assumption for reasonable performance).
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