It is not Hearsay!
Chew, Tip O'Neill IS the witness here, he has direct evidence ONLY of the conversation he had with Powers and O'Donnell.
Tip's direct knowledge of this conversation has nothing to do the possibility that Powers and O'Donnell where telling the truth.
I hope you can understand this.
If we were asking about the colour of the wallpaper in the room where Tip O'Neil had that conversation with O'Donnell and Powers, then yes, he would be a perfect witness. But the subject of the story is what allegedly happened between the FBI, O'Donnell, and Powers. Tip O'Neil did not personally witness that event, he's only repeating what he was told. That makes it second hand knowledge... hearsay.
Again this only concerns Tip's first hand knowledge of the conversation he had with Powers and O'Donnell. Tip's first hand knowledge of the conversation is not hearsay.
You want to claim Powers and O'Donnell are liars go for it, or that Powers and O'Donnell both misunderstood that the FBI did not intend to influence a fabriacted story.
"Tip O'Neill did not personally witness that event, he's only repeating what he was told. That makes it second hand knowledge... hearsay."
One more time, Tip is not making any claim of knowledge of the assassination, he is relaying first hand knowledge of a conversation he had with Powers and O'Donnell.
For instance
If you neighbor says he committed a crime, you would then have direct knowledge of the conversation where the neighbor stated to you that he committed a crime.
You could be called as a witness to this conversation in a court of law because it is in your direct knowledge, you may not know any facts about the crime other then what was relayed in the conversation, that is within your realm of knowledge.