When you fold your cards in poker they're 'mucked' by the dealer i.e muddled in with the rest of the dead cards so you can't tell which cards came from where. This is to prevent cheating and confusion in ambiguous situations.

With experienced dealers and players, poker games move quite fast. It's quite normal for 5+ hands to be folded in quick succession pre-flop. Manually recording who had what cards would be really impractical.

The best place to record the information without disrupting the game at all is in each of the player's hands themselves. Traditionally this is done with either under table cameras and see-through spots on the table, or RFID.

Close range sensors and secure transmissions.

As long as the reader has to be within a couple of centimetres of the cards there's really nothing other players at the table can do to get the data that isn't obvious. It's not like you can lean over the table and scan your opponent's cards with your phone without raising a few eyebrows.

I'd guess the transmission of the card data from the table to the servers for the broadcast wasn't secure. Either because someone involved set it up that way on purpose to cheat, or because they had a 'password123' sort of password on it without thinking.

Have the RFID chip not return its unique ID, but

   encrypt(concat(ID, random()))

or

   encrypt(concat(ID, chip’s internal counter value()))

where encrypt is a good asymmetric cipher.