I'm not Biden it is a colloquial way of saying "I don't believe you". It's used to express skepticism or disbelief about something someone said. The phrase "not Biden(something)" means "to not accept or believe something as the truth."
Joe swears he never discusses business deals with his son Hunter, but I'm not-Biden-it.
Someone who supports President Joe Biden regardless of what he does because their goal is supposedly to achieve "anything but Trump", but in reality they cannot think for themselves and go for the friendly looking old fuck.
A typical leftoid parasite who enjoys the suffering of other people by robbing them of economic opportunities and personal liberty, while at the same time working to force others into compliance for whatever personal fettish the Bidenite has!
Oh wonderful, the Bidenites are using threats of violence again if we all don't celebrate their l;ove of wearing similulated animal appendages in front of their genitals!