Torterra also has meh Sp.Defense and poor Speed, and you're generally more inclined to use special attacks with Charizard given its Sp.Attack is higher than its Attack. Charizard also 4x resists Torterra's Grass STAB and is immune to its Ground STAB. These things (particularly the Speed) would tend to put things in Charizard's favor, although it's not as clear cut as that, as stuff like items and movesets (Torterra has access to Rock moves) and training all are important things that make it not necessarily guaranteed for one or the other.
But like I said, it's still generally more in Zard's favor.
Charizard being the strongest fire type is also highly questionable, as "strongest" (for a lot of things really) is slightly subjective and/or not perfectly clear or not exactly agreed upon. Base Charizard at the very least is certainly not the strongest fire type. The megas are pretty potent, although they are not necessarily the strongest ones either, as there's other contenders for that spot like Ho-oh, Primal Groudon, etc.
Zard is a wildly popular Pokemon, so it's not crazy that a bunch of places would make the claim that it is the strongest fire type in the series. Doesn't really make it true tho.
