I am an optimist, but this is not an obvious truth. Think of a world with inhabitants that always jump to happy conclusions: “This is our hour. Nothing can go wrong now.” In that world, I would look paranoid. “No, you blind Pollyannas! You don’t know that, not totally.”
In that world, I would talk about clinching divisions. “Now,” I’d say, “now it’s truly known. Now you can focus on playoff affairs, at last. But you shouldn’t do so until now.”
But in our world, pragmatism (or cynicism?) is common. All of us work on assumptions–nobody has a proof with axioms and such that tomorrow’s sun will show up, but I plan my day as if it will just as anybody would. Still, I look optimistic if only by comparison with normal folks, holding on to a tiny probability until it too blinks out.
A victory that locks down a divisional championship counts just as much as a quick win in July that looks insignificant. Your rival’s dooming loss counts just as much for it as a marathon back in spring, with all its glib “I’m saving arms, as this isn’t too important now” quotations. But to clinch isn’t nothing: it says that all of that was worth it.