Torah Temimah cites Shevuos 33b.
Rabbi Yose HaGelili says: Scripture says “and he is a witness or saw or knew” — the verse speaks of testimony that is established by seeing without knowing, or by knowing without seeing.
This limits the verse to forms of monetary testimony that can stand on either pure sight or pure knowledge (for example, a loan counted out before the witnesses), and excludes claims that cannot be established in that dual manner.