He didn't say it was better than average, he just said he couldn't say it was worse than average because he doesn't really have a frame of reference because the people he sees probably tend to be people who had worse childhoods than average
(Which, extrapolating, means it definitely wasn't worse than average compared to the rest of his patients. Which I could have already guessed.)

In his defense, objectively it wasn't that bad. Like nothing really awful happened and any "abuse" was fairly mild as abuse goes.