User blog comment:B214/High School DxD EX/@comment-187.209.19.98-20160201021837/@comment-1359322-20160201122131

Ise never transformed into a natural Dragon in the anime, that's the Juggernaut Drive, a power of the Boosted Gear and Divine Dividing.

Tiamat merely changed her appearance she didn't change her race or species so i don't see your point. That's like saying Ladora (Sairaorg's Rook) can change his appearance into a Dragon so he's a Dragon.