See, this is the thing
There is a disconnect between the fans and Oda in regards to the term "rivals". Fans think rivals are the same thing as equals, because that's what it's like in other manga.
But Oda uses the term much more loosely. Kid and Law are canonically Luffy's rivals, they are both far weaker than him, have shit crews, and do more to support Luffy than oppose him.
If Kid and Law are luffy's rivals, I don't see why Koby can't be one as well.
Both Smoker and Koby both parallel Garp. Smoker is a vice admiral who has chased Luffy since east blue and has cornered him several times. Koby is one of Luffy's closest friends, is known as a "hero", is garp's protege, and has Garp's fighting style.
Just like how Kid and Law both parallel whitebeard. Law has an OP DF, Kid is a conqueror, both rival the future pirate king, both are Luffy's friends. WB = Kid+Law, just like Garp = Smoker + Koby.
No character is going to 100% parallel anyone from Roger's era. Luffy is without equal. But to say that Koby doesn't parallel Garp in any aspect whatsoever is denying the manga.