No it isn't. It is about time someone really explained this "self-evidence".
I will try---but I don't know the case as well as the others do.
Jeremy claimed to have received a call from his father, Nevill, in which the latter asked Jeremy to get round to the farm quickly, because Sheila had "gone crazy" and had got hold of a gun. Jeremy then rang the police and recounted the call from Nevill, saying that the phone had then "gone dead", he had tried to call Nevill back, but couldn't get through. The police asked Jeremy to meet them at the farm.
As a result, it has been reasoned that only Jeremy or Sheila could have committed the murders. If Jeremy really did receive a call from Nevill , then Sheila was the killer. In this scenario, it can't have been Jeremy, as we know that he wasn't at the farm at the time, and we also know that he was outside with the police until they and the firearms officers broke in. By then, everyone was dead.
However, if Jeremy invented the phone call from his father, Nevill not having called him at all, then he (Jeremy) must have been the murderer. In this scenario, he must have made up the story about Sheila and the gun in order to frame her.
You will have to make up your own mind as to whether you believe Jeremy did or did not receive a call from Nevill.