I mean there is some credence to that. Because if you go undiagnosed your body and mind will eventually develop coping mechanisms on its own so you can be almost just as functioning as someone without adhd. You'll just have some quirks.
This is what I got told when I got diagnosed at 19. I had no need to become medicated but instead just to be made aware that I'm different for X reasons and these methods could help your coping mechanisms further. For me it was carrying a little notepad with me and writing everything I should remember in that pad. That little pad pretty much got me through college. Then when I eventually got to Uni, I no longer needed the pad to get through it. I could still benefit from it like, but it was no longer a necessity to do so, due to my internal coping mechanisms getting better.
Obviously I know this doesn't always work for everyone, but I still think as a general rule for adhd that it tracks.
You don't learn how to cope with ADHD better if you are undiagnosed than if you are diagnosed. If that was true people wouldn't need to get diagnosed in adulthood because they would've developed the skills already. I need reminders for my reminders, and reminders for when I forget the second reminder and I still fail to do them. And that's not to mention shame and ridicule you receive for "not even putting in effort" when you put in 10x as much effort and still failed. I don't know a single person with ADHD diagnosed as an adult who got anywhere near NT levels of functioning. Actually they're all worse than every person I know with ADHD who was diagnosed as a child. Not knowing why you are struggling doesn't make it easier.