He clearly isn't going to defect to another party. The conservative party manifesto promised a deal in the election so he's not betraying the people that elected him by defying the whip and voting against a no deal.
You seem pretty clued up on the manifesto? Did you miss this bit:
“The negotiations will undoubtedly be tough, and there will be give and take on both sides, but we continue to believe that no deal is better than a bad deal for the UK.”

