When Luffy said he will and that has Faith in him, but ok.
You can't read properly.
There Is a reason why he's many tiers under Luffy right now? Yes.
Does this mean he won't catch up? No. Luffy already said this. "I Will grow stronger everytime"
And Luffy knew he was way stronger than Coby but he said "I believe in you, I know Coby Will!"
Luffy has faith in him that he'll become Admiral first and foremost. That was the biggest context and point of the entire scene:
There's a reason why Luffy says that he should keep the attitude and become an Admiral if he wishes to fight Luffy. There's a reason why the first thing Coby thinks of after Luffy encourages him is about becoming an admiral.
No shit Coby will become stronger, no shit he'll become an Admiral. But if you're pretending he's somehow going to catch up and become stronger than likes of Zoro come EoS despite being many, many tiers beneath and downright low diff material right now, and has been for over 2+ years, you're just setting yourself up for a fat L, mate.
There's a reason why Oda explicitly draws Coby bowing to and talking about how hard it'll be to catch up to not only Luffy, but also Zoro. There's a reason why Oda is making him hard fanboy over both of them and look up to both of them. And it surely ain't to make him surpass one of them come EoS.