So what exactly did he do different from what he usually does? Do improv? Then what exactly did he do different from all the defeated opponents who did improv?
My main argument is Souma didn't do anything to target the copy move. That's not unbelievable at all. People win just because they are better, that's fact. My gripe is Mimasaka's copy move didn't get countered directly, just overwhelmed by lots of other factors (most of them deriving from the fact that Souma is the protagonist and is therefore that good), making the introduction and build up (that trace was an unbeatable move by concept) of the skill a waste of time. It was a plot element that was severely underused in Souma's win.
Aldini also tapped into his experience when he did his improv, but he came way behind the trace dish, losing unanimously. Having more time to "prepare" doesn't matter in improv. That's the whole point of improv, you don't plan it and depend on inspiration and skill at the time (and yes, that includes your X years of experience. I mean, why the hell not?). Does that mean Aldini is just that inferior to Souma, who won unanimously?
If you're saying Souma's dish isn't improv, that's incorrect. He was clearly winging it when he was cooking those beef parts. He even said he was lucky he had that old fashioned grill with him. So cooking those beef parts in those ways was something he came up with on the fly, just like how Aldini came up with using olive oil on the fly. I think the comparison holds up pretty well.
Now if you're saying that Mimasaka wasn't able to predict how Souma planned to pull the whole dish together, then that makes the least sense. Because why? Why wouldn't Mimasaka predict that final dish? He knew about the oxtail stew, knew about Souma's past, knew about the beef parts, and even knew that they were gonna be used as garnish. He even prepared a targeted counter, a time-consuming bacon garnish. Why did Mimasaka think he would win until the judges started tasting Souma's dish..? Why was Souma's dish so different and unpredictable from everyone else's Mimasaka defeated?
He's the protagonist, that's why.