Like I said you you can speculate. Zoro does speak directly him being a voice of reason on the crew is because he talks directly and doesn't sugar coat situations. If Zoro really thought he'd die he'd have said something along the lines of "this will kill me". Like zoro told law " if this doesn't work and I die it's all up to you" showing zoro thinks he'll die to kaido if he can't put him down. I really don't get where you think zoro isn't a direct guy when that's pretty much him 99% of the time.
for sanji haters:
- there is 0% chance that they knew for sure if any of them can survive.. at that moment, both of them knew it's a certain death..
- zoro survive because of plot armor, as sanji also would.. but other crew won't, that's why only 2 of them in that scene
- if there are more people in that scene or if they had share the pain, then it will become "teamwork" scene, instead of "self sacrifice", and will leave less impression and less valuable
for sanji fanboy:
- zoro has more guts and stronger resolve than sanji.. sanji was shaking, zoro not
- zoro took action faster than sanji, knocking him out
- sanji would survive, but he will not stay conscious..
i'm not trying to powerscale them but.. based on that moment, zoro is more decisive and have stronger resolve than sanji.. it's nothing to do with their combat strength..