"Note: STAB (Same-type attack bonus) is considered in the calculations."
But the "strongest offensive pokemon per type" means not the best attacking pokemon for taking over gyms.
For example:
flying type - Charizard is better in offense than Pidgeot.
water type - Golduck is better in offense than Vaporeon.
Edit:
Flareon is better with Ember + Flamethrower, you should play Charizard only with Wing Attack + Flamethrower or Dragon Claw