User blog comment:FairyRaven/Fairy Tail Chapter 470: Hybrid Theory, Review and Commentary/@comment-124.82.62.133-20160126052221/@comment-4829046-20160126104439

Good point, but that is a different case and the OS abilities post-timeskip are inconsistent so it's a gray area. The first time Erik ever showed Poison magic, it was when he used his Dragon Slayer power. If he didn't explicitly use Dragon Slayer ability, then he used Sound Magic. I'm not going to bring in the KotSS arc because it is filler and would complicate things further but even in that, he needed the features before using poison. Anyways, if you wish to believe so, then feel free: the actual matter was not stated anywhere after all.