Portal: No Escape

And how do you know this isn't supposed to be Chell after the first Portal, having woken up randomly imprisoned somewhere then finding the portal gun in her room? Her "test rounds" could've just been verifying that it was in fact a portal gun and still worked the way she was familiar with.

She'd be rather competent in it's usage, wouldn't she?

He's just playing around with people. He called Portal, a game in which you control a gun which can bend space and fling yourself over buildings, unrealistic. I wouldn't put too much thought into what he's saying.