I have never drunk Coke or watched a boxing match, but my impression is that Coke's and Ali's slogans were only able to be effective because (1) lots of people already really liked drinking Coke and (2) Muhammed Ali was in fact a really good boxer.

I think the "real thing" / "Coke is it" slogans were adopted exactly because other companies were making their own competing products that were intended to be like Coca-Cola. So they were aimed at people who already liked Coca-Cola, or who at least knew that Coca-Cola was a drink lots of people liked, saying "That thing you admire? It's our product, not any of those inferior imitations".

So perhaps we can amend CK's comment to something like this: Good marketing isn't about saying "look at me, I'm the greatest" except in some special cases where people are already looking at you and at least considering the possibility that you might be the greatest.

I still don't know whether it's right, though. I would be entirely unsurprised to hear of a product that had a lot of success by going in with a we're-the-best marketing campaign very early in its life.

[EDITED to remove superfluous parentheses.]