Gene Kelly started dancing when he was 8, he was an athlete as a teenager, and was teaching dance when he was 20. His level of experience in the 50's is comparable to competitive professionals of today. Debbie had to somehow catch up, or in her words, get good enough to fake it. Dancing is dancing. Style doesn't change the story. No one else was offering stories of dancers who rose to the top of their class in a short time, so I did, and it is still more of a cautionary tale, than a glowing example.
Also I think you underestimate how good the dancers needed to be, and how demanding the industry was, for film making in the 40's and 50's. Producers hated second takes, it was expensive, and time consuming, so the dancers had to nail each number in one take, which took many hours of practice on tight schedules. Only a handful of male actors could pull this off, and producers and choreographers didn't even like giving anyone new a chance. It wasn't like today's movies where you can get a dozen do overs. Also, good female dancers were a dime a dozen, and they would all kill to dance with Fred or Gene. I dare say, the requirements were more akin to today's competitions. Auditions were like the first rounds of a championship, and if you happen to make it to the final, you only have one take to do 5 numbers to convince judges you are the champion.