When Matias is falling asleep during the "strategy" lecture and Nascimento makes him hold the grenade to keep him awake, do you think he would have given him a real, live grenade?
The pin is only there to keep the 'spoon' in, which is what he had to stay awake to hold down. As long as the spoon hasn't been released, the stays unlit and you can render the grenade safe buy reinserting the retaining pin. Only if the user has allowed the spoon to pop off causing the timed fuse to burn is the only option to throw it somewhere. In that situation reinserting the spoon and the pin wouldn't stop the process and the grenade would still go off.