Matt is ultimately "an ordinary Joe". He's not an actor by profession, he's not reading scripted lines.
However.
He also isn't some unsuspecting chump set by Derren and his family. He's a big Derren fan, he's applied to work with him, he has been through an audition process. This is all concealed from the viewer, but it's pretty important stuff to edit out, isn't it? He's bound to be expecting strange things to happen. He loves Derren, so he's gonna completely go along with it and do as he's told/supposed to (the audition will have identified him as suitable for this role).
If he were now to turn around and say "yeah actually, I just went along with it cos it was Derren and that, I mean, you don't just fall asleep at the click of a fingers and wake up in a simulator!" does it actually reflect that bad on Derren? I doubt it. Derren can justifiably plead ignorance (and point to legions of similar stunts), and say "it's hardly my fault he was faking, how am I to tell?". Derren fans won't believe the guy, they'll say he's just trying to get in the papers, or that his memory's just returning now and making it seem that way to him. (As I said earlier, I was the 'star' of a stage hypnotist's show once, and nobody really believed me when much later I confessed to being conscious the whole time. If nothing else, you just look like a bit of a tit admitting it!)
I guess if all such volunteers (eg. Ben from casino stunt) colluded to blow the lid, that could be a little more difficult to shake off, but it's not particularly likely (why would they turn on their hero?) and Derren is still ultimately in the clear.