That has nothing to do with his ability to debate. That has to do with his cult following. To his followers, it doesn't matter if Christie wipes him up on the debate floor. If January 6 wasn't enough to turn their back away, do you think losing a debate to a candidate will? I doubt it. Do you think calling people names is really good debating? Little Marco and low energy Jeb. It was effective first time around, but his shots against Christie come off lame and juvenile.
Christie is by far the most effective debater in the field. That won't matter absent a federal indictment. And who knows? That may not sway Trump voters either. But it does give Christie and other opponents much more ammunition.