Because he has characters in the series say they are equal and was genuinely surprised when people thought otherwise. To me WOG is greater than everything. The entirety of most of the discussion here is just using elements in the story to try and tell what the author intention is. But those interpretations can be dead wrong. That's why when you have direct statements from the man himself telling you the intention, That's what should be prioritized. I don't fully say Kirin is the strongest for the same reason I don't take Ignia at his word when he says he can casually beat Acnologia. Self hype is the most worthless kind of statements imo as it's mostly ego talking. What I do believe is that Kirin is a contender for strongest and he could be slightly stronger than the other ddsk given his seniority and respect Misaki gives him. But not to a significant degree since all the ddsk were introduced as being as strong as Suzaku.
Depends on how much POE they can access. Natsu's DF alone wasn't enough during that fight against Zero and was quickly overwhelmed after the initial power up. It was only in combination with max FOE that he won. If Gray and Erza were similarly given a temporary power up and then used POE to the max, then yeah they could have beat Zero.
But there is no contradiction. Lack of action doesn't = anti feats. Let's look at what Gray has done in 100 yq. He lost to skullion. Not an anti feats as at the same time Natsu lost to Madmole. Mashima's terrible for not fitting in a rematch, but the results of the other fights and the common trope of "fairy tail doesn't lose to the same enemy twice" tells us that Gray would have likely won if they rematched. He stomped the raijinshuu. Winning a fight easily isn't an anti feat. He beat Mirajane offscreen. Winning a fight isn't an anti feats and it being offscreen doesn't tell us anything about how much power he used. Did he leave base? Did she leave base? The world may never know. (As a side note I don't get why people scale Mira to just skullion level when she was in base Satan soul the whole fight. That's like seeing Natsu lose to Suzaku then saying that Natsu in every form is one shot material to him. Doesn't make sense to me.) Gray beat the god seed along with Juvia while in base. Winning a fight isn't an anti feats. Not much needs to be said about Elentir. Him and Natsu were easily taken out at the beginning, then Gray came back and beat the opponent. Winning a fight and being stated to be as strong as Natsu isn't an anti feat. In the labyrinth he washed Sai. Easily winning a fight isn't an anti feat.
Now let's look at Erza. She lost to hax in round one against Kirin at the same time Natsu lost then came back and overpowered her opponent with one of her weakest armors. Winning isn't an anti feat. Then she fought Laxus to extreme diff and both passed out. Having an extreme diff fight against someone who's stated to be stronger than Natsu isn't an anti feats. Lost to Yoko at the same time Natsu lost to hakune. She came back and handily beat Yoko. Winning a fight isn't an anti feat. She got taken out by Suzaku at the same time Natsu got taken out. Getting beat in a low powered state isn't an anti feat. She then beat Misaki who's stated to be as strong as Suzaku in the same arc that Natsu only managed to come to a draw with Suzaku with Suzaku having the upper hand. Beating an opponent equal to someone Natsu didn't beat doesn't somehow contradict her being stronger. Gildarts hasn't done anything the entire series but he's still relevant because the statements put him so high. Natsu fighting the big bad is nice and all but it never meant that he was the strongest back in the og series and it doesn't mean that now. Especially when the big bad have to be nerfed to be closer to his level.
Portrayal wise everytime Erza and Laxus are mentioned they are implied to be stronger. Everytime Gray and Natsu are mentioned together, they are implied to be on par with each other.
. He also has characters in the series one shot fodder fullbuster.
