"likely a fabrication"
"with zero evidence otherwise"
The burden of proof is on you to show that the quote is a fabrication, not on the journalist.
Let's go through it, though.
Going to jail for conspiracy and obstruction of justice makes him a criminal, but not necessarily a liar on techniques the U.S. government used against counterculture.
Secondly, Nixon and his administration did outrageous things. You can hardly dismiss more allegations of outrageous acts as more-likely-to-be-fabricated because it fits pre-existing narrative. That's, like, the opposite of how priors work. "It fits exactly what you want to hear deep down about two bad people: Ehrlichmann and Nixon" -- YES! Nixon and Ehrlichmann were bad people and did bad things. Did they do what Ehrlichmann (allegedly) describes? Who knows, but it isn't an argument against their having done something horrific that they had other horrific policies.
Nixon, you may remember, bugged a Supreme Court justice.