I'm saying that verse 6 could be based on a real event, but unlike the 1988 appearance we don't have photos to check if the appearance looked like how Jesus was supposed to look.
I think my explanation is better than Richard Carriers at least:
Then He Appeared to Over Five Hundred Brethren at Once! • Richard Carrier
I think he is saying it was just a mass hallucination
I think the wording of the article discredits itself. The Gospels have been dealt with by quite a range of people, Christian and not. That they are inerrant, perfect witnesses is an assertion of faith, which even I don't believe. That they are worthless, however, is at least as hard to support. Almost no atheists who work with Christian history believe that, even if they don't accept the reality of the Resurrection.
As I noted above, I think there's a kind of ambiguous nature to Jesus' resurrected existence. But hallucination is not a credible understanding of Paul's words. He gives us a sequence of 4 events: death, burial, resurrection, appearances. So he sees the resurrection as an event separate from the appearances. The other three events are objective, public events. The natural reading is that the resurrection is too. This does not establish the empty tomb, but it does imply some kind of resurrection event before the appearances. Something like the stories in the Gospels seems like the obvious form of that.
That there's a specific resurrection event is also implied by his discussion of the End. Jesus' resurrection is a proleptic form of his final coming. Also a specific event.
One of the most interesting things about Paul's citation of the early creed is that it doesn't mention Mary, who was the first witness according to the Gospels. If Paul differentiated between the resurrection event and Jesus' appearances, perhaps he considered Mary's experience at the tomb to be part of the resurrection. Mark seems to make this distinction. It describes the women as part of the resurrection story, but leaves the appearance to Peter and the disciples as a separate, future event, to take place in Galilee. (Matthew agrees. Luke and John, of course, show appearances to Peter and the other disciples immediately. I think 1 Cor 15 gives support to the version in Mark and Matthew.)
It's also worth noting that even liberal scholars take seriously the early tradition that Mark is based on Peter's preaching. I find it highly improbable that Mark would have added a non-existent empty tomb to that.