It's real, but only in the first and second editions.  (I don't have any electronic proof, but I've seen it in my copy of the second edition and someone else's copy of the first edition.)  It's the only exercise in the chapter.

The full quote in the second edition is:

> Take any book on homological algebra,
> and prove all the theorems without
> looking at the proofs given in the
> book.
> 
> Homological algebra was invented by
> Eilenberg-MacLane. General category
> theory (i. e. the theory of
> arrow-theoretic results) is generally
> known as abstract nonsense (the
> terminology is due to Steenrod).

If I'm not mistaken, the quote is the same in the first edition.  First edition: page 105; Second edition: page 175; so you can look in your library to see if I messed up the quote!