User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-5894890-20160926140328/@comment-84.197.239.101-20160928133125

If Erza's another enchanted sword, that may explain her aptitude for swords :D

Still, I doubt this. The Irene would simply have to dispell the enchantment to dispose of her or rewrite her in order to fight for Alvarez.