can you say why it doesn't fit in the timeline?
Looks to me like it does. They were around that area at around that time.
I can't find a reason that it doesn't, and the witness and his friends say 100% it was him. (2 of them are Dr's. Well educated)
Several reasons. You can check the timeline at the beginning of this thread. and see that it is unlikely.
If the couple had traveled through the entry station into Yellowstone NP, that would be known to authorities (and it would mean that, inexplicably, they travel back to Spread Creek in a remarkably short period of time just to camp in the same place again. I believe we'd know if they had gone to Yellowstone NP. Or if Brian had gone without Gabby.
The meeting seems to be at night - so what happened next? Brian drove the long way around back to Spread Creek? Or did he drive the long way around (to avoid detection at the Park Entrance) both times? That's 6 hours round trip.
Where in the timeline do you think this could have taken place?
Gabby and Brian arrive Grand Teton NP August 25. Van is at Spread Creek August 27-29, with Brian hitchhiking and using the ATM at Colter Bay. around August 29-30. August 31, Brian buys gas in Benton (which is in Arkansas, I believe) and Sept 1 he is back at home.
Oddly, the man in the bar who has inserted his views into this case doesn't seem to know exactly when it was that he himself was in West Yellowstone - if anyone has that date from another article, I'd appreciate knowing it. He also doesn't describe Brian at all well and seems like an unreliable narrator. Anyone who has studied serial killers (like many people here on WS) knows there's no "serial killer" look.
Since Gabby is still alive in Jackson, WY on August 27 (last known sighting of her, Merry Piglets), and we know they didn't drive to Yellowstone that day (van is seen by multiple people and captured on camera at Spread Creek), and Brian made it back to FL in remarkable time (after leaving Jackson/Grand Teton - not Yellowstone on or about August 30, I do not find it at all plausible that he'd drive out of his way, away from home, adding hours to his trip, around the edge of Yellowstone and not through it, in order to go to a bar (it's said that neither of the couple drank) and chat with some man, who then somehow remembers him.
If Brian did do this (and there's no way he could have evaded cameras at the exit to GTNP or the entrance to YNP), this "witness" needs to show when he was in West Yellowstone. No way to explain Brian's actions from August 27-30 (ATM fraud, hitchhiking Colter Bay area - twice) or the federal warrant for his arrest, or his rapid journey home.