I think after having a history of decades of chapters to build up to various fights, you should probably give Morikawa some more credit.
People can't complain Morikawa keeps dragging the story and doing endless buildup, then complain there isn't any buildup.
Ippo can do it. The Dempsey roll is inherently neither orthodox nor southpaw, neither is the peekaboo style. He doesn't use jabs at all, or any other type of punch that specifically requires a favored side. He's trained to use all his signatures on both sides, and done so in actual matches. And yes, his continued self-training and internalization of other fighters is a factor.
Also, I am certain they will bring up that not being a natural south paw, Ippo is only 80~90% of a match. It's still the best option they have. Since when did this manga go for "silver bullet" solutions? Never.
But yeah just lazily handwave that all away yourself, why don't you.