Because the Cancel (or even a Dispel) only creates a temporal wall of sorts. It is not a spell designed for foiling scrying, but it can work to hinder it. And since it uses an already existing mechanic, that means no special rules are required. Also remember, Cancel Essence, even in its normal use, does NOT guarantee that it will stop incoming spells. It isn't a perfect solution, but it is an aid for those who wish to try and avoid after-the-fact detection.
And yes, those other things are all options for breaking the temporal trace links. Again, none of them are perfect, as higher level seer spells can still work to pick up the "scent" again.
