Be mindful that you can harbor ill feelings toward someone and put on a smiling face in public to get through whatever they gotta do. Politicans are knowns for talking shyt in private, while acting different in public. And vise versa.
Trump has been telling his supporters that Biden is every negative thing under the sun, but he also said that when Biden called him in the spring of 2020 to give him advice on how to handle covid, Biden was very nice to him and was a nice man. Biden still thinks Trump is a threat to Democracy just like all the military dudes and government officials who spoke out against Trump said he is. But Biden has a job to do.
Biden is a traditionalist. The tradition of the President is that he welcomes the incoming President to the white house to talk about important topics both domestic and international. The President also ensures a smooth transition and peaceful transfer of power. Even though Trump didn't do that for Biden, Biden isn't a petty person that looks for revenge. He'll attend the inauguration, put a smile on his face, look like the biggest cuck in the world and go back home.