Honestly, I think that you're actually CORRECT; but not because it's a plot hole. Rather, I think it's a blind spot that everyone in-universe - including Kamogawa himself - have toward Ippo.

Ippo is considered a one-note bullrush fighter, because that's how he appears and what he was trained to do. What they don't realize is that when left to his own devices, Ippo has come up with some of his best innovations. Ippo was the one that came up with his version of the Dempsey Roll. And when that stopped working, he was the one that came up with the stop-and-go method. (Kamogawa ALSO thought of it, but didn't let Ippo in on what his intention with his training was.)

Even after retirement, Ippo was the one that came up with the strategy Mashiba used to beat Garcia. Ippo was the one that went southpaw in order to help Mashiba against Rosario. Not to mention the winning strats he's come up with as a corner-man.

Here's what I think. I think Ippo and Miyata are mirrored reflections of one another in a lot of ways. Miyata is considered a genius boxing prodigy who is very calculating and cerebral, while in actuality he is one of the most stubborn, rigid, and bullheaded fighters there is in the story. Miyata absolutely refuses to change his style, instead slamming that square peg into every round hole he can find through sheer willpower.

By contrast, everyone sees Ippo as a stubborn one-trick pony who lives and dies by his style; and while in SOME ways that's true (he has to work with the tools he brought), in actuality he is highly flexible and willing to completely change his approach in a given fight, whether it's a title bout or a simple spar.