[mass noun]1facts, information, and skills acquired through experience or education; the theoretical or practical understanding of a subject:
Pete Sampras has high level practical understanding of tennis. Tennis expert is a person who is very knowledgeable about tennis. Therefore, Sampras is an tennis expert.
Let me do one.
Pete Sampras has high level practical understanding of tennis. An expert tennis historian/statistician is a person with an extremely detailed knowledge of past tennis matches, who has a proven track record in understanding trends and making accurate predictions of future outcomes. From the fact that Pete Sampras has a high level practical understanding of tennis it does not follow that he is an expert tennis historian/statistician.
Do you disagree with anything in the above reasoning?
But, my point is that every reasonable person will rather believe Sampras than anonymous posters on MTF.
Every reasonable person has to decide for themselves
what is true or not. Even if you choose to defer to an expert, you're the one that decides what qualifies someone to be an expert, and of course different 'experts' disagree.
Ultimately all experts have is arguments based on facts and data. You should believe someone because their argument is sound, not because they have some magical mantle on their head. In that regard, people on MTF can make sound arguments too, and saying "but you're not an expert" does nothing to invalidate those arguments. Knowledge is a tool that can be used by everyone, not just those sitting atop some high tower throwing down scraps of wisdom to the peasants below.