Sanji was saving Momonosuke, not fighting, King was the one pissed cuz he Sanji lowkey saved Momo passing through Kaido's Commanders, Sanji was chilling and got surprised by his Full Zoan (he fought Marco with that, see chapter 1005)
Works basicly the same way here.
And now this chapter proves my point once again with Sanji vs King and Queen + Perospero sneak attacking