He's only pretending to think he won the election to delegitimize Biden's win

First, there is no legal basis for voter fraud or any sort of major illegitimate voting that could have impacted the outcome of the Presidential election. In multiple lawsuits, judges have fully dismissed "baseless" claims of voter fraud. Additionally, any possible recounts have such a large margin that the outcome of Joe Biden as President is likely to remain the same. This means that top Trump advisors must reasonably understand that Trump claiming the Presidency is a flat-out lie. However, while he ultimately is going to have to concede the election, Trump's rhetoric is causing a national partisan divide, which will draw power away from Biden's goal of unifying the country and dampen his ability to create any meaningful national reforms, as nearly half of the American public will view him as an "illegitimate President."