The bit about contracts not being input valuation is no different than the case for assertions. It's basically saying that if assertions were inappropriate, so are contracts. So if you find that like blurry, the blurriness predates contracts.

That said, the distinction is generally pretty clear: an assertion failure (and therefore a contract failure) indicates a bug. It must never happen, and therefore it must always be OK to suppress the validation. If you see it fire, that's a bug report, and your code needs fixing. And if your contract to the caller is that you will enforce some constraint on them (as opposed to assuming they already took care of ensuring it), then you need the validation in your runtime code.