Yes it says all it needs to say. It’s an elliptical modernist narrative. If you need to have your hand held then watch the Doris Day/Rock Hudson vehicles. If you want to see what 2001 would be like with more ‘explicit’ storytelling then watch 2010...it ‘explains’ everything. And it’s long been forgotten.
I think the ending of 2001 is really a litmus test on the viewer. It asks whether you can handle a little vagueness and thinking in your movies, or need it all spelled out. Basically should you see art house movies or stick to blockbusters. The whole movie is really an exercise in the moviemaker meme of “show don’t tell”, what little dialog there is in the movie is in the movie is largely tangential to the plot, and the ending takes that to a rather extreme level. But if you’re willing to think about the movies you watch after you watch them it all makes perfectly good sense. My funniest 2001 incident came when I bought the DVD and on the way went to my wife to share my joy of now owning it. She ask “what’s it about” (she’s not a nerd, of any kind really, I married out of clan), which lead to a rather prolonged pause on my part, and finally “I guess you could say evolution”. She still hasn’t watched it.
Guess what it's about?