> The fact that Wigner doesn't know that yet, changes nothing in reality. > The actual 6-photon experiment is a bit too technical for me

There's a more approachable explanation, and lengthy discussion, over at Physics Forums: https://www.physicsforums.com/threads/a-realization-of-a-bas...

I'm no expert, but it seems to me the issue is of a different, more fundamental, nature.