Originally it was "achievements" and now you're talking in terms of upsets though. What even qualifies as an achievement? Would continued dominance count, perhaps back to back to back (etc) titles in a sporting league with much more parity? Is it only an achievement if it's across a single season?
I think it just becomes downright silly to call it the greatest across all sports across all time. I don't think such a thing can even be objectively known. It's amazing and Leicester should be celebrated for it, but there's no sporting calculus known to man that enables us to rank such things.