Statement of the obvious. But here's why.
He didn't offer a sensible, alternative explanation of why he paid Cohen the money and why it was described as a reimbursement. Nor did he have a single witness to support such an alternative explanation.
He ran out of people willing to lie for him and he didn't dare to perjure himself.
Every actual witness and all the documentation told the same story, which was the story the prosecution presented.
Assuming the jury agrees, and finds Chump guilty, then he'll appeal for various reasons, of course. Delay being his usual method.