I would imagine more than half of those kids are playing in a game that is extremely out of their comfort zone. You're basically pulling a group of strangers together, running a simplified scheme that's a bit different than what many of them are used to, and hoping that they build up enough chemistry together in practice over a week's time to put a decent product on the field.
I would almost argue that the practices would tell more about the prospect's ability than the actual game itself. Certain positions, such as corners, receivers, and defensive linemen probably are at an advantage over position groups like QB, safeties and linebackers in those games. Running backs, I'm sure, are probably lost in pass protection during those games. You love a game like that if you're a CB prospect-you just line up in man and do your thing.
So, not to make excuses for McMillian, as I'm sure other LB's have stood out in those games before. But he'll be coached up by some of the better minds in the business, and I'd look more to his overall body of work as a better indicator of how he'll perform; not an isolated game, where he was probably out of his comfort zone.