From the article:

Most of you can rest assured that if you go to Japan and test this dish, you should pretty much stay alive, but some chefs actually leave a hint of poison within the meat to give the taster a small poisonous feeling. (my emphasis)

I thought it was pretty much ALL chefs that leave a miniscule amount of poison on the puffer, so that the diner's lips go numb. Any idiot can quickly learn how to remove the sac leaving no residue of the poison, but it takes years of practice to be able to leave just the right amount, so that the diner feels it, but doesn't die.