To be fair, the movie has been out nearly six months and we only clocked it because of a post on Reddit, so there's that.
Matt Damon is no stranger to making a cameo appearance. Just look at something like 'Thor: Ragnarok' or 'EuroTrip' or, arguably, 'Interstellar' where his role was completely missing from any of the lead-up promotion to it. Damon, it turns out, also had a minor cameo in 'Deadpool 2' alongside Brad Pitt as The Vanisher. Hell, Terry Crews was in 'Deadpool 2' and nobody clocked it until the movie was almost released. Even Ryan Reynolds was getting on the cameo business in HIS OWN MOVIE.
So, in the scene where Cable - that's Josh Brolin's character - travels back to our time, he confronts two rednecks and quickly shocks both of them and takes their truck. Those two guys? Matt Damon and Alan Tudyk.
So, as it turns out, Damon was happy to go under a few hours of makeup and completely change his physically shape just for that one scene which lasts of all of maybe five, ten seconds. Alan Tudyk, as well, you don't even recognise the guy either. That level of commitment to a joke, the same with Brad Pitt literally turning up for one scene and one joke.