User blog comment:FairyRaven/Fairy Tail Chapter 363: The Stories That Demons Read, Review and Commentary/@comment-1153699-20131207173113/@comment-7418318-20131212000918

" eventho it was earlier stated he could not eat it."

It was never said that Natsu could not eat Laxus lightning. What they said was that Natsu got sick "from eating" the lightning. Not from the attempt, but from actually eating it.

So it was already established that he could eat it right at the end of the Tower of Heaven arc, but with him getting knocked out so quickly by Laxus, he probably didn't get the chance to use that absorbed lightning and for some reason it was just temorary (maybe his powers weren't matured enough back then for a permanent absorbtion).