Everyone is entitled to their opinion. Maybe he had a bad experience previously by not finding unseasoned nori introduced it to his tank upon recommendation from someone else and it killed livestock. Who knows what his reasonings are, but you are getting advice from people also that say you can use it.
Personally it's cheaper than buying the fish processed seaweed, and this is made for human consumption. I'm not sure other companies can say the same thing about their seaweed/algae.
So just in case you get hungry per say, you can eat it also.