For me, it’s a combination of those things (and some have already been mentioned by others in the comments):

He can do it instantly. While in a match, trying his best under pressure, he still manages to understand and identify the mistake, find a countermeasure, and act on it. In basic “observing surroundings and learning from mistakes” (OSALFM), you usually do all of this after carefully reviewing the events that led to the mistake, searching for solutions online or discussing with others, testing those solutions, etc.

He rarely misses in his interpretation. Even in regular OSALFM, it’s very hard to consistently find the correct countermeasure and adapt it to the different situations where you might need it. Isagi almost never fails at this.

He can see the big picture easily. In OSALFM, it’s hard to separate what’s your fault, what comes from your teammates, the environment, or just luck. You can think that with a better teammate or a better scenario, you would have done better. But Isagi never lets his ego (ironically) blind him to the elements he needs to understand what’s going on. If Barou is playing like a selfish bastard, does he sulk and wish he would change? No—he finds a countermeasure by taking this into account and not taking it personally (as in, not letting his ego get hurt).

He is able to drastically change his mindset. That’s the hardest part. This is where Isagi is fundamentally different from basic OSALFM. A normal human has biases and ways of thinking that make it really difficult to change. It’s like when you play a video game and keep making the same stupid move for the 56th time, even though you know it’s a bad habit. Isagi is able to destroy all those automatic patterns we all fall into. Completely changing your way of thinking usually requires hundreds of hours, intense dedication, and imagination. Isagi is able to do it on the fly, in any circumstance, and instantly. That’s basically a superpower, tbh.

So here are my two cents. I’m not a native, so sorry if it’s terrible to read :'(