When I read it I assume that it is a retort to very similar hysterical sentiments to the ones we see today. I don't read it as a sub swimming being as ridiculous as a computer thinking, but rather that the question "is that machine swimming or not?", much like in the robot walking example, isn't particularly valuable.

We don't break a sweat when we say that a robot is walking, because we don't care about walking. We haven't internalised it as the final frontier of humanness. I read the quote as saying that whether a computer can think or not should be as pointless a question as whether or not a robot can "walk".

I'm intrigued enough now to try to hunt down the context.