Sirhan is likely lying about that because it doesn't sound great to acknowledge that he had complete responsibility for his actions.
Likely that he's lying based on what specifically?
What has he gained from lying about it for decades?
It didn't prevent him from getting the Death Penalty for his crime. It so far hasn't gotten him out of prison.
So his story is that he can't remember. Very convenient. Even if there was another shooter, and there is no credible evidence of such, Sirhan was still an assassin.
The medical examiner refused to conclude that Sirhan fired the shot that killed RFK. There is strong evidence that more than 8 shots were fired (Sirhan couldn't have fired more than 8 ).
No one is arguing that Sirhan is innocent. He was there with a gun in his hand as you noted. But the physical evidence suggests that he wasn't the only one shooting in the kitchen that night.
The fact that there was a conspiracy doesn't exonerate Sirhan unless it can be proven that someone manipulated him psychologically. Which there is circumstantial evidence for (Sirhan is easily hypnotized) but likely isn't possible to prove short of confessions by the individuals involved.