That's not true in any meaningful sense and points to the fact that the author hasn't any clue what they're talking about. Which doesn't magically invalidate their experiments and data, but it does mean you should probably take their opinion with an extra table spoon of salt.
DES of course is reversible, it's an encryption algorithm, so if you have the key you can just reverse it, that's the whole point. But what DES-crypt does is something else, First it takes the DES algorithm and tampers with the internals, it has 12-bits worth of random data for this, a salt (today obviously we use larger salts, but this one already makes time-space tradeoffs painful, 4096 times worse than normal against a "good" DES crypt). Then it encrypts a bunch of zero bytes using the password, and then it encrypts the result, and that result and so on, 25 times in total.
This isn't a reversible algorithm. You can't take some encrypted DES bytes, and then run DES backwards 25 times to find out what the key was knowing the original was all zero bytes, you quickly get conflicting possibilities.
What you _can_ do, in reasonable time, is try all the plausible passwords in turn, for a particular salt value. This is what DES crackers do, and for passwords like "ksdcunfm" it works pretty well on commodity hardware, which is why we don't use DES Crypt any more.
But that's not reversing the algorithm.