Hi Roselli,
The good news about buying from Costco is they have a great return policy so if you don’t like the Smartsilk mattress pad then you can return it for a refund.
I hope you have the chance to share your comments and feedback about it when you receive it.
If it has an actual membrane then it would be similar in terms of performance to other semi breathable water resistant membranes that are used in other mattress protectors. Most of them are made from polurethane but there are also other types as well that work on similar principles.
There is some more technical information about different types of membranes in some of the links in post #2 here but knowing the actual manufacturing details of the membrane would be less important than how it works “as a whole” on your mattress 
Phoenix