Even if you can make a valid claim King is tiers above Queen in terms of feats and narrative, it doesn't matter for Sanji, because he'll will always be in the conversation because Oda designed him to be.
Sanji can fight weaker enemies, even significantly weaker enemies, and still eventually catch up in terms of power.
I think Queen being significantly weaker (lets say 2/3 tiers at most) than King would not be a bad thing , because it adds nuance and makes the world feel more real, otherwise every set of opposition for Zoro and Sanji would just be a copy and paste of Kaku and Jabra
Copy and Pasta Kaku and Jabra, copy and paste their doriki levels, and then reiterate them and you have King and Queen. This is boring.
Sanji says essentially that he was worried about being taken over by Zoro and Jinbei, and this could fit in the narrative that somewhere along the line, Sanji could have a notably weaker enemy than Zoro and simply not progress enough and almost be left behind (for example queen), and then make that kind of statement, only to be given the opportunity for signifcant growth in elbaph and solidify his strength again.
I don't see anything wrong with this, it goes in line with jinbei's bounty being higher, it goes in line with sanji's worry.