You normally would have to go through two security checks and a standard boarding check to get on that flight. Maybe he got out on the tarmac and somehow just walked to the plane.
It doesn't mean that you will never ever succeed (that swiss cheese model mentioned elsewhere), but most probably (at least in western countries and modern airports) you will be caught quickly and go straight to airport jail. Or if you somehow make it then walk into airplane which is actually fully booked, probably same result.
That being said I once had a very stressful situation in the middle of the night in Mombasa, Kenya where I was changing flight to Tanzania, but directions where to go within airport were vague, random staff didn't have a clue and when I arrived at the door to buses the staff/stewardess just told me to wait. I waited and waited till departure time, they didn't care at all about me and my increasingly urgent questions. I raised this with some random airport staff who was just running around, he just yelled at that staff and took me running straight through runways in heavy rain to the correct flight which was about to depart with aircraft door already closed. This was 2011. Impossible in the west, maybe now even there.
I bet if you try this, you’d have a >3% chance of succeeding… and probably >33% with a vest, no hand luggage, and actively talking on the radio.
Dress as law enforcement, and you’ll have a 100% success rate.
Different people check different documents for different reasons. I’ve had a police passport check post X-ray at Manchester in the past, the U.K. doesn’t typically do exit checks, preferring to rely on API. Schengen does, but sometimes they don’t stamp you on the way out which can be a right pain^W brexit bonus. Then there’s all the loopholes, like Brussels-Lille on Eurosta, where I believe you can get a schengen exit stamp but never actually leave schengen (two tickets from Brussels, one to Lille, one to London, use the london one, get the stamp, but then leave at Lille with your Lille ticket. Perhaps done with an accomplice if the Lille ticket needs stamping at Brussels, I’ve never done that route before.