We don't. No one said that. Did you read even a single line of his post?
Being strongest of any group simply means being stronger than anyone else in that group. It has nothing to do with a specific thing. You just beat the shit out of anyone else in 1 vs 1. That's what Zoro describes it as. That's what Oda paints it as in Vivre Card.
Why would being a stronger "bamboo user" give Vergo an advantage over Smoker's logia property? Did you even read the CONTEXT of what he said?
"Turning into smoke cloud is pointless against someone with stronger haki! It will only make you a bigger target!"
Because haki is a critical factor that is main reason why anyone is strong of any fighting type.
Same as why Kaido thinks of CoC first thing instead of sword skills when Zoro cuts him using 9 sword style:
Haki matters more than skills for any style.