It was meant as a joke. If you know enough mathematicians you know what I mean, but maybe it's even some deeper truth in it, because you have to be open to just play around with ideas and puzzles. It's precisely what's killed when thinking math were like it is presented by Bourbaki. Bourbaki presents the final state of a mathematical subject but doesn't tell you anything about how you get there. In this sense it's like a very good encyclopedia to look up mathematical facts but a lousy textbook, because it's not telling you how in fact to do math.