He could be pandering to the conspiracy that Obama is the shadow president, although this doesn't make sense as Biden and Obama are quite different on certain issues. Either that or he assumes that Biden is going to drop out at some point so he's treating it like he's running against Harris.
He's been saying for months that Obama is the shadow President. So this is more of that. Trump has had an obsession with Obama since 2011, and I truly think he genuinely believes that Obama is pulling the strings and using Biden to "persecute" him.
