He's not really "guessing" the card type, there's a pretty standard algorithm called the Luhn Algorithm that all credit card companies use when issuing their cards.
The algorithm is public domain and all it does is tell you who issued the card which doesn't seem like an issue to me
It isn't providing a visual for all credit card issuers, though (eg Diner's Club, JCB). Those cards may still be processed correctly (I don't have any way of verifying), but visually treating a correct input like an invalid input isn't going to help conversion much.
Easily solved by simply adding treatments for more card types, and styling the entire card isn't that much more effort than just displaying an issuer logo.
Where this starts to get really interesting (from a slick + branded perspective) is when you start handling gift cards.
The algorithm is public domain and all it does is tell you who issued the card which doesn't seem like an issue to me