So he claimed, yes.
But the "benefit of the doubt" isn't necessary here. The theory that CIA recruited Oswald specifically to assassinate JFK has been conclusively proven wrong. The magic bullet theory has been conclusively proven wrong. That the USSR had evidence that the CIA assassinated JFK has been conclusively proven wrong. This has been studied in tedious amount of detail.
A little too convenient, don't you think?
Ruby was not afraid of the consequences? He couldn't wait for the judicial system to pronounce the death sentence? (which was very likely if Oswald was indeed the killer). If he loved the president so much, didn't he want to know if there were others involved, so they could be punished as well?
I think it is very unlikely that Jack Ruby's actions were borne of impulsiveness.
Jack Ruby was once again let to die in a prison before he could testify in a proper court. So many convenient ends disappear...