There's a couple of reasons this can be explained, none of them necessarily any more likely or relevant than the others.
1. Jim's writing changes. I doubt anyone (even Jim) would argue he has been consistent with some of his rules over the years. While Jim likes to pretend he has a "hard magic" system, it's actually quite plot based. Which all of them are of course, it has to be. Everything is just a tool for the story. But for it to be believable for the reader it has to makes sense and be fairly consistent. For this story it seems he needed Marcone to be a Denarian and a magic user. It's also clearly a future part of the series and he felt he had to set it up now. Yes, it appears to break the rules. But likely Jim could find a good enough explanation if challenged.
2. Marcone has had the Coin of Namshiel since SmF. Who knows how long he resisted (at least in his mind)? Likely, he saw the business opportunity and he took it. But that's 6 years later or so that he reveals it to Dresden (and perhaps the world). It is a small amount of time even if you are a rare talent to become that good...but then again look at Ivy. She isn't just a big tank of magic power - she has so much knowledge that clearly allows her to master her powers and puts her on a league seemingly beyond the Senior Council and even the Ladies. Even if Namshiel isn't providing the horsepower perhaps he knows the right secrets - places to draw energy from outside yourself, ways to be extra efficient, secrets that are normally forbidden. Plus Hellfire which grants quite a fair bit of magic muscle on it's own.
3. The breakdown of reality caused by Ethniu and the Eye of Balor. Between a Titan showing up (which was probably hard enough on Reality, plus several old gods and demigods etc) and her firing off her father's Eye, reality was having trouble playing by the rules. Dresden's magic (by his own account) was far stronger and more "real" than it should have been. Ferrovax was doing his best to hold the barrier closed between the Nevernever and the mortal world. But it was still very thing and likely Things slipped through (note - I love that Ferrovax says his contribution to the defense of Chicago must be subtle and then goes and closes the Earth and the Nevernever). Anyway, Marcone probably had far more juice than normal (and unlike Harry he wasn't spent) when he fought Ethniu. So Marcone might not be that strong normally.
4. He might have found a cheat way to gain more magical muscle. A sacrifice, a bargain, a special object or potions, a ritual or spell etc. Even if he hasn't he still might.
Measure Harry's strength by the bodies of his enemies. He's changed the nature of a Fallen's Shadow, survived death, killed a whole species, forged a bond with the Caretaker of the prison of the Gods, imprisoned a Titan, caused the death of two Faerie Queens and fought and won against three Outsider nobles. Smoke that Carlos. You got you butt handed to you by an Ice Princess.
Couldn't agree more.
Harry is sometimes portrayed weaker or less skillful but as I said above part of that is Jim's narrative reasons and part of that is we only see it from Harry's perspective. Harry might even think some wizards are better than they are. Harry also is dealing with his own misconceptions, preconceived notions and insecurities. Any and all of that skew his view from the "truth".
