I think that by sophist you mean skeptic. A sophist is one who missuses the art of dialectic to confuse the person listening to him rather than to arrive at truth. On the other hand, Skeptics are those who deny the human's ability to have knowledge.
Skeptics cannot deny that humans have the ability of understanding. This is because you have to understand the skeptic's argument to be a skeptic in the first place.
Now in the meno, Plato treats knowledge as an object of knowledge. So he is asking whether you can KNOW KNOWLEDGE or have knowledge about Knowledge rather than having Knowledge about an object of experience. Knowledge is presented as if it is an object of experience. Obviously your understanding grasps what knowledge means but its very tough to lay out what you understand of it.
Plato has his recollection theory where the soul already experienced everything that there is in a "Previous world" and right now we are only remembering. So knowledge comes from withing.. A good teacher is one who makes you remember what you already know. He will "prove" that when he teaches the slave geometry. he'll cheat though lol