Origin Form Dialga VStar - Weakness not taking double damage?

Sakura-Burst

I played a match using Charizard EX deck, and the opponent used Dialga VStar. The opponent took 2 prize cards, so burning darkness base damage would go up from 180 -> 240. The Origin Form Dialga VStar shows a weakness of fire, but it only did the base damage of 240. Is there something wrong with the weakness to fire in this case?

uncomonfossil

Charizard ex changes type to Dark

TechHog

That Charizard ex is Darkness-type, not Fire-type. Read cards.