I think they would never say that, because they are quite careful not to ever try to enforce any kind of implementation/performance detail. They just care about the interface and the external results, any timing consideration always excluded.

So the way they seem to do it, is just to open a 8 lanes wide road to performance in front of you ("if you compute that, it WILL be useless and dropped at some point. Now compute what you want."). My poor english skills would not have called this a "hint". I guess this is just a minor misunderstanding.

Of course I'm assuming here that computing those excess rows has no side-effect. Hopefully this is always the case?!

Agreed for sure.