Yi and Gangplank's Ult=Invincible?

In one of my recent rank matches a Yi on the enemy team somehow manage to stay within my team's fountain for more than 10 secs trying to kill our AFK to get a penta kill, without the fountain defenses even activating to stop him. I think Gangplank's ult had something to do with it since he activated it the same time Yi step on to on fountain and remained there for a long period of time.