On the whole, if as an adult you get as far as an assessment, then you're highly likely to be diagnosed because anyone who doesn't have it will self filter out before they get that far. It's a tedious process and an expensive one if you go privately, and surely not many people would go that far unless they were convinced it was needed.
Psychs are paid (by NHS and privately) to assess not to diagnose - there's no extra money if they give a diagnosis or not.
But yes, I know some people who got an assessment and didn't meet the criteria for diagnosis, it does happen and is sort of the point of an assessment.
Sometimes the assessment points towards a different condition with crossover traits, sometimes it's that the symptoms are trauma based, sometimes it's none of the above.
Most of the time the person seeking assessment gets a diagnosis because they have correctly identified that the symptoms match what they have struggled with all their life. For me it felt like I'd finally found the missing memos or textbook pages for being a human properly that I'd always known I didn't have, but didn't know why.
It would be nice if the media stopped perpetuating the myth of "paying for a diagnosis".
Good luck with your journey OP.