I was curious because I didn't think they were charmable either. But I found this from Aug 2000.
"Enchanters level 49 or higher can charm fire giants with Allure, although the casting time of their charm makes it nearly impossible to charm a fire giant that is beating on them (unless it is slowed) - two enchanters trying to charm a single giant generally will work, as one of them tanks it and the other gets Allure off uninterrupted."
http://everquest.allakhazam.com/db/zones.html?zstrat=50