Is/was the AmEx card in question one of the contactless versions? The only way someone standing next to you could have stolen the card info would be if it was that type of card. Mag stripe or chip requiring you to insert the card into the terminal cannot be compromised that way. Also, unless AmEx specifically told you that is how the card info was compromised, you will never know. It could have been a skimmer on a pay at the gas pump, it could have been practically anywhere mainly with physical contact with the card.
Paper security sleeves will do nothing to prevent RFID scams. The sleeve must have copper or aluminum or some other similar metal in it and be designed for RFID protection. It is called a Faraday cage when it can protect your card.
Most money belts also do not offer any additional RFID protection being a simple silk or nylon single layer of cloth. The ones I have seen with claimed RFID protection are bulky, heavy, and very uncomfortable to wear.
But actual RFID theft of credit card info is not a problem compared to other easier ways for thieves to get your card info.