Because he is quick to condemn what Begum did (which was unquestionably more severe) when she was fifteen, citing her age as no excuse, but seeks to downplay racial abuse from himself at the same age, partly because of the age he was when he said it. He’s mentioned the antiquity of those actions, as well as the fact he was at school at the time, in order to minimise them on several occasions.
If he was to simply say, I said things I shouldn’t and I’m sorry, then he wouldn’t be a hypocrite on the issue. Instead he deploys his lack of maturity as a deflection tactic as a means to try and downplay what he said and the impact it had on others. That makes him a hypocrite to a material extent, especially as he is a party leader.
And like Begum, he is also treacherous. Talking money from Russia and saying the UK is like North Korea in Congress will have had a more profound negative impact on the UK than anything Begum said or did. The difference is that his treachery is ongoing.