A lot of panniers have some sort of water-proofing or resistance. Just because Sierra's website doesn't say that doesn't mean they will sponge up moisture. Check out the product at Deuter's US site:
http://tinyurl.com/2n25qx

The description of the fabric, Super-Polytex, includes this: "Made from a robust 600 denier polyester thread with a PU coating...."

So it has some sort of protective coating. Regardless, buy them and waterproof them yourself:
http://tinyurl.com/2u5vvn

Those packs look nice!