Driving on dirt isn't at all like driving on tarmac.
Those Sprint Cars are more like powerboats... you need power to change direction. Simply let off the gas or hit the brakes, you've got no bite, you'll go in a straight line.
I've looked over the slow motion replays... what Tony does is get the car pointed left (possibly with the throttle) right before he hits Kevin. Kevin has an "Oh 🤬" moment right before he gets mowed over when he realizes he's run right into Tony's path. If Tony had gunned the throttle before hitting Kevin, he'd be sideways at that point already instead of pointed at him. As it was, it's very likely Tony didn't realize he was there until too late.
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Scientifically, there's a big reason drivers hit motorcycles. It's because they're very narrow objects. When they're crossing your field of vision, (and, remember, black suit against a dark track), they don't register all that well compared to the cars around them. Many drivers literally cannot see the cyclist until they've hit them (which is why lane-splitting is so dangerous... and why bikers have to "act like cars", staying center-lane, in traffic, so they're in a place where drivers normally look).
Tony has several cars in front of him. A bright blue car has just swerved on the track and Tony is likely wondering why. By the time it becomes clear there's a dark object dancing across the track into his path, he's got very limited options.
I'm sure I'm not the only one who's played "guess which way I'm going" coming towards a pedestrian on a dark road. It's terrifying. Now imagine doing that while driving on ice, coming around a bend, with a helmet on. That's pretty much what happened here.