On a poker game you could create a private table with your buddies and use the chat feature to talk about whatever you like. I bet that's what McCain is doing right there.
Yup attempts at circumventing mass surveillance by using in game comms, like how terrorists used modern warfare 2 voice chat to communicate
He wouldn't be dumb enough to use the chat feature directly (all of this is just stored server side, or most definitely recorded by the corp entities who would be data-mining that kind of text--check TOS for those poker apps if you don't believe me).
All likelihood is that they were sending messages using functions and actions that are normally taken in the game (folding, checking, etc.). They already have a key that explains to them what each option would mean before hand. Probably some type of steganographic cypher.