User blog comment:IamJakuhoRaikoben/Chapter 245: The Unbranded Man, Review./@comment-3987368-20110730184547/@comment-3987368-20110730192546

Yes, he's eaten it before, about one year before Lucy joined the guild I believe. He's now aware of the fact that he can't eat lightning (or at least more cautious about eating it, I hope).

If he's implied to have eaten it when we couldn't see, then I'd agree with you about it. But for the reason that he knows eating is bad, I'm inclined to think he absorbed it differently.