Because that article is not very clear. At first I read it as the Obamas were going to their house. But thinking more, it could be they were all due to attend the same Hollywood event somewhere else. The article does not say.
And there is the 2 hour call with Eric Idle before they were murdered. So Bob was on the phone, when they were supposed to be at a function with the Obamas ?
Actually, I just tried to do a timeline. Saturday night the party argument. Sun afternoon the Idle call, Sun evening bodies found, Sun night son arrested.
So it would have been a Sunday evening Obama event.
Fixed my head now and given myself a talking to. However, the right wing conspiracists will be hard at it.