The lance was a downward orientation held in front of him, so all he'd have to do is move his arm back to get it into a throwing position. I would think someone could do a throwing motion in that length of time. He'd also simply land on his feet. Your mech was training his weapons on where he'd land, so if he was aiming I wouldn't think he'd be wildly moving the gun around, making it an easier shot and your weapon seems like it was a relatively large target and wasn't very far.

I'll edit the post to clarify his stance, although it would have to be in my post before last.