No, it’s not the exact same concept, not even remotely. You’re diluting haki to prop Zoro up. Stop.
Luffy’s hands are surrounded by a haki bubble. When he hits something with the bubble, the bubble eats away at what it touches from the inside out. Zoro does not do that with his slashes. He simply flings them and it cuts. His application of haki on that is normal CoA. It’s not what Luffy is using. It’s not about contact that makes it advanced, it’s what the haki actually does.
Zoro hasn’t been using haki since Alabasta. Stop trying to retcon the story by applying standards for twenty years later to the beginning parts of the story.
It’s not a poor comparison. The bullet is the projectile which would be coated in haki. The slash is a projectile which would need to be coated in haki.