Sirhan is in prison. He needs to appear to be sympathetic to ever get out. He is not going to tell the truth and say that he is an anti-Semitic terrorist who targeted RFK for assassination due to his support of Israel.
Sirhan has never denied his views about the Middle East where he was born.
In fact, initially, some Arab groups wanted Sirhan to embrace the Palestinian cause in terms of his motive in order to bring more attention to the plight of Palestinians. Very little attention was paid to the Palestinians in the US in the 1960s.
The ONLY logical explanation for him lying by saying "he doesn't remember anything" is because it protects potential co-conspirators. He doesn't have to answer questions about that if he maintains that he doesn't remember anything.
But I believe he sincerely doesn't remember because he most likely was drugged and/or under hypnosis at the time of the shooting.
He is trying to get out. Not add time to his sentence. This is a trick as old as the hills. Prisoners claim to get religion, can't remember what happened, were under the influence of drugs or alcohol. It's a way to deflect responsibility for the underlying criminal act.
Can you cite other examples of prisoners saying they don't remember committing the crime while pleading Guilty to the crime?
I'd find your argument more convincing if Sirhan plead not Guilty.