Genetics allow you to get in shape FAST. However there are many examples of people who trained for years and got way more out of their potential than others.
I think it's simple statistics. The last 10% of ability is based on training consistency. Some extreme talents can get to 90% very fast. Those with less talents have to work their way there through thousands of miles. These "blue collar" guys also lose fitness they worked for a decade quite quickly relative to a blue chip.
I think powerlifting shows us what the body can do. Guys who weren't built like Greek gods in high school can become world class power lifters (talking about the drug free ones) through years of work. Person#2 may never squat 750 or bench 600, but they could squat 650 and bench 500. Hence the 90% gains I'm referencing. Person #1 who squats 750-800 and benches 600 may be able to walk into a gym 10 years off training and throw 400 up. Person #2 may walk in the same gym with the same time off and find his body acts like it never trained.
Genetic potential matters, but only at the HIGHEST elite levels. Unfortunately talent between the 2 levels usually decides who gets the luxury to maximize genetic potential and who has to pursue other endeavors.
Disclaimer: though I'm sure it's blatantly obvious. This is just theory based on observation.