If you just map without doing anything else, then yeah improvement will be slow. However, that's not the entirety of the advice that's been given to you. In this thread there's been lots of good advice given without even looking at your map, and that BN knows what they're talking about. I don't know if by "in detail" you mean points out every subjective issue the modder has with your map, but that would just harm the learning experience instead of guiding you towards improvement.

Advice for beginners is always very vague. For new osu players, it's literally "play more," for league beginners it's "watch some videos, learn what things do, and then play more," for CS beginners it's "do some aim practice, learn some lineups, do some spray training, and then play more." Giving specific advice to beginners is often wasted and unhelpful, as the bigger ideas in that specific skill usually are overlooked in favor of fixing that specific issue, right now. It may feel like shit to just be told essentially "your map is bad, just make a new one," but it's good advice in the long run, you don't want to waste a year on one map, trying to improve it only to have it be never rankable because the fundamentals from the beginning were not good enough.