You also have no idea of the last 13 years were due to something else, none of us really do. This disease can get better or worse for years. Unless your doctor is measuring something before that was bad and after supplements that improved it then you have no clue.
It is problematic. The longer the time frame for improvement, the more other potential factors creep in that could also have contributed to mentioned improvement. If one would feel a benefit straight away or within, say, at most a couple of weeks, it could be reasonable to attribute that to the supplement. I generally don't take any new drug or supplement I trial for longer than 2-3 weeks because any improvement that hasn't come in that time period gets harder to judge and most things I try make me feel slightly worse from baseline anyway, so it wouldn't be worth the pain going through things for months on end.