So to say "It is meaningless..." is to point out that perhaps you are being misled by these words, that they make you imagine a use which they do not have. They do perhaps evoke an idea (the prolongation of life, etc.), but the game with the sentence is so arranged that it doesn't have the essential point which makes useful the game with similarly constructed sentences. (As the 'race between the hare and the hedgehog' looks like a race, but isn't one.)That follows this, which helps understand what he's talking about (because it is what he's talking about):
Suppose I have invented a medicine and say: Every man who takes this medicine for a few months will have his life extended by one month. If he hadn't taken it, he would have died a month earlier. "We can't know whether it was really the medicine; or whether he wouldn't have lived just as long without it." Isn't this a misleading way to speak? Wouldn't it be better to say: "It is meaningless to say this medicine prolongs life, if testing the claim is ruled out in this way." In other words, we are indeed dealing with a correct English sentence constructed on the analogy of sentences which are in common use, but you are not clear about the fundamental difference in the use of these sentences. It isn't easy to have a clear view of this use. The sentence is there before your eyes, but not a clear overall representation of its use.