You're leaving out or don't know what Campbell said he was told. I've read what Anarchy has above there, but for me it's that they or Allen told him that you can't have two players report in as eligible. So, no, they didn't forget or mess up. Nor does what they told him have anything to do with two players reporting in as eligible on the same play. What's left? What's left is to interpret what that statement means with respect to the very play it applies to! I'm sorry, but you either comprehend that fact or you don't.
What did we see, who are the two players? Obviously, Skipper & Decker. What were they both doing? They both did things to suggest they were reporting in eligible. Which is okay, if both were attempting to report, but we know that isn't what they were doing. The Lions wanted Allen to recognize only the one! We don't know, Campbell addressing the crew meant nothing, the six of them aren't who players report to. Accordingly, he finds Allen, after which , could be that Allen is unsure & makes a call. This is what I believe the evidence supports most.
Campbell said he spoke to Allen before the game. There's no rule against multiple players reporting as eligible on a single play, nor is there a rule (that I'm aware of) preventing multiple players approaching the referee but only one of them declaring as eligible.
Essentially what you are asking us to believe is that they had a conversation about this potential play before the game, Allen told Campbell they couldn't do what they were planning, but Campbell went ahead and did it anyway? Again, Occam's razor leads me to believe the ref just made a mistake - something they're known to do from time to time - as opposed to a convoluted situation where Campbell is fabricating the conversation he had with Allen, or was told he couldn't do what they wanted to but tried anyway, or something else.
Again, I don't pretend to have any idea who said what or when. However, in one of the breakdowns I saw with Dean Blandino, he mentioned that if a coach in a pregame review meeting outlined that 3 players would approach the ref with the intent of only one of them reporting as eligible with the other 2 used as pawns to try to distract the defense, he was pretty resolute in suggesting that the referee would have told Cambell to have only the eligible player report.
It's possible Campbell could have drawn up the ALIGNMENT of the play with X's and O's for the refs to review, illustrating that the player on the left side of the line was the one he intended to be eligible in the unbalanced line. But Blandino felt the 3-card monte element involving multiple linemen was most likely left out of the discussion with the officials.
Blandino acknowledged that he wasn't at that pre-game meeting, but he said the crew would not promote a pre-play diversion to try to deceive the other team. I get it, as the former head of officials, he's going to take their side. But he took offense to the suggestion that Campbell had fully spelled out what the Lions intended on that play.
I'm actually surprised that the media hasn't followed up on exactly what was reviewed and discussed before the game. That would add another layer to help understand what actually happened that resulted in the debacle that we all witnessed.