Let's say there is a player who knows nothing about Sicilian Defense. If they always start the game with 1. d4 or 1. c4 as white and play the Caro-Kann as black against 1. e4, it seems to me that they can theoretically be a player at the top level without knowing anything about Sicilian. Of course, they will have a problem coaching others, but I do not see why zero knowledge of the Sicilian can prevent them from being a top-level player.
Wilhelm Steinitz: I have never in my life played the French Defence, which is the dullest of all openings.