As they wrote (and if you're that far into it you really should have been reading) in the paragraph before that, that's the source of the bug in modern OpenZFS

As they wrote (and if you're that far into it you really should have been reading) in the paragraph before that, that's the source of the bug in modern OpenZFS but that doesn't mean it was a bug at all in 2006 Sun ZFS. A lot of changes have happened since then, and there may have been something else in ZFS back then or in Solaris itself that meant it wouldn't ever be hit. It's a genuinely interesting tidbit that a little change like that 17 years ago could pop up this way, there is nobody who'd somehow hold Sun (long dead) responsible let alone whichever team did it (even if one dev made the change it should have then needed review/clearance, and if not that's still the problem of Sun not them). Kind of a strange thing for you to zero in on out of a whole interesting article.