This is a terrible reason. Sanji or Zoro could beat the Pre-TS SHs no diff just like Kuma did.
I agree with you on that point that Sanji and Zoro are stronger than the pre-timeskip crew.
However, I do not think that Sanji has surpassed Ivankov in strength yet, especially after seeing the gap in their power pre time skip. I also dont see zoro having surpassed kuma in strength yet, let alone speed (Which kuma outclasses them both in badly). Ivankov alone was shown to be a versatile tank, able to not only use a versatile arsenal but also surviving attacks that would have killed any other character. His speed is also insane being fast enought to create multiple after-images of himself and in the anime (vs megellan) showed reaction feats on par with gear 2 luffy.
Kuma also has some serious speed advantages over the SH Dou and though they are closer to Kuma than they were pre-timeskip there is still a huge difference in strength. Kuma could destroy islands with a single attack and one hit KO Giants. Also, In CQC if Ivankov touches either one of them once and pumps them with hornones its game over.
With Ivankov and Kuma together I don't see Zoro and Sanji winning. Separately, yes I do think they (Zoro+Sanji) could take down Kuma, and possibly Ivankov with High or extreme difficulty, but together(Kuma+Iva) they are too much.