Stuttering can relapse if you put it in a long enough term like I know a female star's stuttering 'disappeared' for almost 10-20 yrs but when she was pregnant her stuttering came back and it was very severe(I can't remember her name, sorry). You gotta guarantee it will never relapse for a lifetime and that can only be called 'cure'. Since you have realized it may be not a cure for anyone, then it is not a cure. As for the part about how you can prove fluency to others, of course SLPs have professional methods of evaluations of your speech fluency but imo if you can guarantee for a lifetime you can't be recognized as having problems with your speeches by average people then it can be considered as 'cured', but even we define cure in this way there is still no 'cure'.