There was construction that day at the terminal and 305 couldn't hook up to a jetway at that gate, thus why they had to board up the aft stairs. When it was time to board, they cued up and once they gave their ticket to Williams they walked down a walkway that led to a door which opened up to the tarmac. They then walked a short distance to where Flo was standing at the bottom of the stairs checking their boarding passes.
So Mike's scenario doesn't really work in that scenario. That would require Cooper to somehow be OUTSIDE before any other passengers. It's highly unlikely that Williams would have let anyone just walk down the gate ramp and out onto the runway. So I don't see how Cooper gets out there for this interaction to take place and then ends up back into the terminal in the regular cue.
Second, that would not have been an interaction that a stew would have forgotten once they realized he was the hijacker and certainly would have ended up being part of their narrative during their testimony.
So much has gotten mixed up in the intervening years. For example, many passengers, including Michael Cooper, thought Al Lee was FBI because you'll see them say in interviews that there were FBI agents down by the plane. In hindsight that was a reasonable deduction on their part but it wasn't true.
Larry Finegold got something very mixed up and it ended up in Bruce's book and caused a whole bunch of confusion. Larry told Bruce that an FBI agent he knew (Larry was a US attorney, famously) came ONTO THE PLANE and they exchanged a laugh with each other. Well, we know that's not true because there is actual video footage and a photograph of Larry running into this agent he knew (it was Don Steele according to Detlor) and this interaction happened when Larry entered the terminal with the other passengers. You can see Larry in the footage smiling and nodding his head at an agent.
So that's a very clear of example of something occurring but being misremembered. I suspect that what Michael Cooper probably saw Hal Williams coming outside to ask Flo if they were ready to let passengers onboard yet. As I said, it was a bit of an ad hoc arrangement due to the gate construction, so as the gate agent Hal would have needed to go outside and talk to someone from the crew at some point.
I bet that interaction between Flo and Hal somehow over the decades morphed into this being Cooper. This story doesn't appear until an interview Mike gave in the 2000's.
So it's not that these people are inventing things out of whole cloth, but just conflating and misremembering.
Although sometimes they absolutely embellish, such as Finegold stating in a media interview in the late 70's that he got a really good look at Cooper and that his law enforcement training as a prosecutor gave him the chops to really take note of the man's appearance. Well, of course, we have Finegold's 302 and he says he never saw the hijacker a single time.
Jack Almstad claiming that he cracked a joke with Tina and Cooper while waiting in the aisle for the lavatory doesn't ring particularly true with me either.