There is something called sarcasm and if you really think Sanji would die, then your IQ is lower than Zoro's.
RS is not that durable, Katakuri beat up Ichiji in his RS without a problem using only his fists/legs and here you have King trying to use his Zoan form and his beak to ''kill'' Sanji. Sanji surviving it will show you how durable Sanji is by tanking an attack meant to split him in 2, not like a cerstain someone who would have fainted by a little stab.