But it is not shown that his beak tip is as small as his finger, lol. Thats an exageration from you.
People can die from holes in their stomac even from a blunt attack, if the attack is strong enoug, if Luffy punches a weak dude in stomac like he means to kill it, be sure that his punch will go straight in his stomac.
A blunt attack can do crazy damage, especialy if is from a dude bigger 100x time than you.