(I posted this earlier, but took it down when the EPA stuff started breaking).

So why did Biden start using "ULTRA"? What are both sides discussing?

We have already seen MK ULTRA getting brought up in movies and TV shows:

https://www.express.co.uk/showbiz/tv-radio/1616646/Dr-Brenner-Stranger-Things-real-person-Netflix

It is being planted in the public mind.

https://www.politico.eu/article/the-secret-history-of-fort-detrick-the-cias-base-for-mind-control-experiments/amp/

MK ULTRA is real. It never really stopped. That's just a story. It took on new forms, but the government found new ways to use it.

Gottlieb searched relentlessly for a way to blast away human minds so new ones could be implanted in their place. He tested an astonishing variety of drug combinations, often in conjunction with other torments like electro-shock or sensory deprivation. In the United States, his victims were unwitting subjects at jails and hospitals, including a federal prison in Atlanta and an addiction research center in Lexington, Kentucky.

These were the most gruesome experiments the U.S. government ever conducted on human beings. In one of the them, seven prisoners in Lexington, Kentucky, were given multiple doses of LSD for 77 straight days. In another, captured North Koreans were given depressant drugs, then dosed with potent stimulants and exposed to intense heat and electro-shock while they were in the weakened state of transition. These experiments destroyed many minds and caused an unknown number of deaths. Many of the potions, pills and aerosols administered to victims were created at Detrick.

Nonetheless Fort Detrick, as it was renamed in 1956, remained Gottlieb’s chemical base. After the end of MK-ULTRA, he used it to develop and store the CIA’s arsenal of poisons. In his freezers, he kept biological agents that could cause diseases including smallpox, tuberculosis and anthrax, and a number of organic toxins including snake venom and paralytic shellfish poison. He developed poisons intended to kill Cuban leader Fidel Castro and Congolese leader Patrice Lumumba.

They were threatening that if ULTRA is exposed ( a comm for the child trafficking and experimenting) they'll turn Trump into the equivalent of King Richard. (Who have written as a deformed, child-killing monster).

Trump has responded with a "try it" and has chosen to embrace the title. He'll take their insult and make it a compliment.