Regardless Sanji was still trying to stop P1's attack instead of receiving it and p1 managed to easily break through his guard and send him flying. If Sanji was so superior to p1 that he could oneshot him like you're proposing then Sanji's block would've been enough to Halt P1's attack. Aside from that Sanji Landed more than one Kick on P1, so it's not a one-shot regardless
His kicks were non technique btw. And he kicked only two times before the oneshot, a normal kick in base form without Haki, and the second one with RS, though he didn't use the function of RS to increase his speed because the point of that scene was to show us the stealth function (he used Haki here btw).
And no, you can be superior to someone and that someone can still manage to send you flying if you only block. Example: Luffy vs Kaido.
Force of a DJ technique >> DJ normal kick >> common technique >> common kick >> a block. And add Haki to this, the gap is even larger considering it.