This is the way they make a living. Find some chump who lost money who will hire them as lead law firm. They will book 50% of the recovered funds and the rest get pennies on the dollar. Any time a stock loses a substantial percentage you will find the lawyers hovering.
Even though he "knows" it will be a yes vote, this gives him an out by saying he's just following the will of the people. Then no blame can be attached to him in the future. He's a game player. His expertise is in game theory. Varoufakis is the author of several books on game theory (according to Wikipedia). It's just a hoax. Of course if they vote no then they are in a world of hurt.