If someone actually said "the best thing since sliced bread", then yes, it's almost certainly pure hyperbole and if they meant it literally they have only themselves to blame when people don't realise that. But if they said something more specific, such as calling someone the best actress since <some actress who is actually within living memory>, it's not so clear. There's probably a general hint of "comparable to" or "merits mentioning in the same sentence as" in there, at least.