Because nothing suggests he can't hurt King? This defense that Sanji failed to get past is weaker than the weakest version of King we've known, and Jimbo didn't even have flames on.
The difference b/w Zoro and Sanji is, Zoro proved himself against Kaido before failing against King. So he has a better portfolio than Sanji. Everyone and their mothers with a half-working brain already know Zoro's AP shits on Sanji's, and by extension, we also know Sanji can't get past King's flame defense.
Zoro only landed one KoH attack on King and that put him down. King withstanding the other two CoC attacks from Zoro proves he is that good even outside his defense mode. However, we've been selling people this idea that King is a glass canon in his non-defense mode and that anyone could beat him as long as his defense is off. This chapter proves someone of Sanji's caliber couldn't damage him even if he attacked him with the intent to kill him outside his defense mode.
We all used to think most people could hurt King outside defense mode, but this chapter made it very clear it's not an easy business. This hype coming from Sanji is more ridiculous because you can't block Sanji's attacks just by being physically tough, you have to resist the heat too, which is something even Kaido couldn't do. All of this is news for all of us.