That sentiment is meant to appeal to people who had previously never read about Angela Davis, the BPP, The Black Radical Movements, or the FBI's efforts to neutralize them.
Many of the figures from that era were accused at one time or another of being agents. Often accused by people who are afraid of their own shadows and who wouldn't ride when it was go-time.
The accusations are an extension of the efforts to neutralize them, because decades later the same forces want to discredit them and what they accomplished.
Freedom of Information Act would turn up any documented snitching or cooperation by any of those figures.
But random person speculating on Twitter knows who their audience is, and what they will believe.
It is what is.