No, just because he said it doesn't mean it is. If he called that pitch a knuckle ball, that doesn't make it so.
The pitch he threw looks an awful lot like a curveball and very little like a slider. He may be holding the pitch like a slider, but it is coming out looking like a curve. He wouldn't be the first guy to do that. That is why guys often throw slurves. The two pitches are close enough in their execution that you can try to throw one and get the other, or something real close to it.