Yes, I have 3 Amex cards that are transparent. The chip is clearly displayed.
I’m not claiming it’s ironclad proof of the security of RFID cards, merely evidence. Banks have a lot to lose if the cards they issue have a reputation for being defrauded; if it happens a lot merchants will stop accepting them, and consumers will stop carrying them. Given this, it seems likely to me that before banks started issuing RFID cards, they probably spent a considerable sum of time money researching the issue and concluded that there wasn’t any new risk that was significnat enough to bother mentioning to cardholders.
I can’t comment on that specific wallet. Several years ago I bought this one and it seems to work. I had an opportunity to play with it at an Oyster topup machine at an otherwise empty tube station in London. The machine could detect Oyster cards in my companion’s regular wallet, but not in my RFID blocking one.
I have an RFID key card to open the secure doors at my place of employment.
When I place the key card in an ordinary leather wallet I can just hold my butt close to the receiver by the door and the door will open.
When I place the same key card in a secure foil lined wallet I am not able to open the door.
So yes the wallets do block RFID signals.
As an aside, I used to do that too - a buddy claimed that the scanner actually scanned *butts because no two were alike 
(*well, ok he used a different word)
If you buy an RFID-blocking wallet or sleeve, read reviews first. Some seem to work while others don’t.
20mm is kinda thick. I was hoping it to be the size of a cigarette holder.
I used to use the Umbra Bungee case. It is about 1/2 the thickness of the other aluminium wallets and holds up to 6 cards. I can’t speak to how well it blocks RFID but some reviews speak to the fact that security cards have to be removed from it to work.