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.
1: a phrase used by someone declining or calling out the bullshit they're being told,
or
2: another way to say "don't complicate something/give it to me straight"
or
3: similarly to use 2, it also could be used as a replacement for "not my problem.
example 1: DUDE DID YOU TAKE MY FUCKING WALLET?? - "dude im not for the shits, go harass someone else"
example 2: I like this girl but she likes this other guy and now this other girls flirting with me, what do I do? "fuck that, i'm not for the shits, figure it out on your own"
example 3: okkk so idk if I like u or not but I might like u but IDKKKK. "im not for the shits, youve been unsure for 3 days, ive been patient long enough"
A derogatory term of something or someone that is not pure. The evil that you see lurking in reality is not a conspiracy. The more known word describing this conspiratorial perceived evil a.k.a. "Notnilc" - instead it's more fittingly called "Notmilk". Notmilk is something that doesn't resonate with the Christ consciousness. Notmilk is someone that cannot introspect to metamorphize an inner philosopher's stone.