The only thing we can infer from this dialogue, if it is legit, it's that Killer is more lethal than Kamazo.
The rest is a speculation, Zoro doesn't know how much stronger Killer would have been.
He is just being confident.
You are wrong.
You can't just give Killer those blades, weapons or whatever he meant and then ignore everything else about that battle. I'm seeing too many people do this and that is a mistake.
Zoro was then and still is much stronger than Killer.
Zoro is physically much stronger than Killer. He stopped Killer from pulling the scythe out of his body by just flexing his muscles.
Zoro is much faster than Killer. Killer was only able to hit Zoro because he was distracted by a character he was ignoring and didn't consider a threat.
Zoro also blitzed Killer. Zoro didn't need to block or break through Killer's weapon to hit him.
Zoro still had faster attacks. His lion song and dragon blaze attacks are two of his fastest attacks.
Zoro is a more skilled fighter and blade/edge weapon user. He used the scythe skillfully in an instant and used it to help defeat Killer. Killer had more experience with it and time to train with it.
Zoro vs Ryuma is a excellent fight that strong how strong Zoro with 2 swords can be.