Well, it looks that way because you just want to hate Sanji and doesn't want to think it through.
Strength wise, Sanji is at least stronger than 1 Supernova at Sabaody, it made him eligible in this aspect.
Age wise, Sanji is younger than a lot of Supernova, this one passed.
Storyline contribution prior, combat wise, Sanji face a similar opponent of Zoro's level.
As for plot relevance, what he did in PreTS & postTS is more impactful than a lot of Supernova.
He's eligible on many criterias.
The only reason he isn't one is that he's not as famous as Zoro in East Blue (in-universe explanation) and making SH have 3 Supernova is too much/too OP, doesn't fit the underdog/struggling element of the main cast (meta explanation).