It's absolutely defamation if they have no or unreliable sources and something Reiner's son could sue over. They are a big enough publication to know the risks here.
They'll reveal those sources to a judge if it comes to it. They won't reveal them to the public because nobody wants to have their name attached to something like this.
It could still be false, but I somewhat doubt it is.
That's still a terrible way of evaluating credibility, especially when a determination of defamation is not the same thing as a determination of truth.
Meh. Information is often jumbled and wrong in the immediate aftermath of a newsworthy event, and it is tempting to accept tenuous claims which reinforce one's biases. Take the murder of Bob Lee, in which early reports were a bit off and convinced maaaaany people it was a street crime (confirming their biases about San Francsisco).
There's no real advantage to accepting PEOPLE's claim at this point. It's possibly wrong, and we'll probably know the truth in good time.