Just like a Lib to miss the point completely. The point Cheney was making is in no way diminished by the fact that they may have had some passing interactions in social contexts. The point was not whether they had ever met, but that they had never met on the senate floor. Leave it to libs to try to confuse the situation.
That is the appropriate after-the-fact explanation. But the fact of the matter is that the VP overreached. It would have been just as effective to say "I've never seen you on the Senate floor." But, for dramatic effect, he went for the "never met you until we walked out here," and it was not true.