You misunderstand the relationship between IATA and its member air carriers. IATA is nothing more than a trade group for which its members pay dues. It sets certain general standards which air carriers are free to adhere to. By way of example, ticketing. But, IATA is not a government and it does not make the rules.

However, air carriers are free to set their own limits for what may be contained in checked luggage and make a choice to disregard their trade group's standard if they wish.

Arguing with a front line employee of an airline about policy and IATA policies gets you nowhere.