If you are not good at talking about tennis it does not mean that you are not very knowledgeable about tennis.
True. But the argument here is physically knowing 'how' to do something and an having an understanding of how what you're doing works
are two different things. I can tie my shoe laces very fast, and have been doing this nearly my whole life. Do I have any knowledge about shoelace tieing? No. If someone asked me how best to tie their shoelaces I would have no idea what to say to them.
Sampras is an 'expert' at playing tennis. That doesn't mean he knows anything about tennis history or understanding statistical trends, or dealing with a myriad of unknown variables. It doesn't even mean he follows tennis currently! If I wanted an expert on those things I would probably ask a professional gambler, who makes a living making predictions, not hitting the ball over the other side of the net.