In Oda's mind, he probably thinks Sanji's post-timeskip development makes sense judging by the way he grew during the timeskip. He sent him in an Okama island for 2years, with no woman in sight, creating this result
He even pointed out himself that Post-timeskip Sanji was worse than preskip Sanji in this aspect
But i think he exaggerated this, turning this running gag into something substantial to his character to the point of even affecting the plot
As far as power-level is concerned however, i can understand your feelings. Pretimeskip, Sanji used to fight and defeat one of the top3 strongest antagonists in major arcs like Alabasta(M0, M1, M2), Enies Lobby(Lucci, Kaku, Jyabura) or participating in Monster Trio group fights(Oars in duo battles with Zoro alongside rest of the SHs, or Pacifista battle), but overall i always perceived that as Oda wanting to portray Sanji as one of the top3 fighters in the crew, but with Luffy and Zoro as stronger.
They were his inspiration from the beginning
With Sanji reiterating in TB that he wants them to fulfil their dreams first and foremost
Oda never wrote Sanji with the intention to be as strong as them, but to be strong enough to be part of the top3 in the crew fighting main antagonists. The issue with post-timeskip however is with the involvement of alliances
Think of it of arcs like Skypiea where we had an ally as powerful as Wyper, but it becoming recurrent post-timeskip ever since the initial involvement of SNs turned out to be worst generation in the battle against yonkos, starting by Law's involvement since Punk Hazard
With the growing importance of worst gen characters, it made Oda increase their panel time and importance against main antagonists. He had to make Luffy and Zoro incorporate the group because of their dreams(Pirate King and Worst Strongest Swordsman) and as a result, with them conserving their importance despite that fact, unlike Sanji's, which regressed to the point they are now portrayed as the leader of the next generation