I'm not sure I believe Mathews at all. First of all, he was talking to Frank Luntz, the republican pollster, and they talking about California's election. Mathews wasn't talking to a politician. It seemed frivolous to me, and the conversation was about voters surprising pollsters.
Luntz predicted Wesley Clark would win the South Carolina primary. He said South Carolina republicans could cross over vote in primaries, there were lots of military voters in South Carolina.
Mathews told Luntz he had voted for republicans in the past, Luntz basically ignored the comment, and moved on to finish a response he had begun. When Luntz stopped talking, Chris repeated, "I have voted for republicans, and one of them was Bush." Luntz looked stunned, then Mathews said, "I did surprise you, didn't I?"
So I'm not so sure Mathews wasn't being flip, just joking around. One thing for sure, it was such a throwaway line, Mathews could certainly get away with claiming it was, to the ghost of Tip O'Neill.