I’m going to guess that the general issue being referenced regarding synchronization is addressed earlier in that section - it’s the implication I’m reading into the way that last sentence is phrased. (I don’t have the book so I can’t look it up - you could probably ask him directly, I believe he provides contact information in the book.)
I’ll go further and make the conjecture that the synchronization issue arises when two people are submitting posts at nearly the same time to where two entries with the same slug are attempted to be submitted. If that slug field doesn’t have a unique constraint on it, you could end up with two posts having the same slug. If you do have a unique constraint, you’ll need to handle the situation of the exception being thrown on that duplicate entry.
This is where I go back to my previous response that it’s probably not going to be a problem for 99+% of the people doing this. Unless I knew I had a very high level of activity in areas that would likely cause slug-conflict, I’d be willing to apply a unique index on the slug, and in the case of the error, return a very generic error message to the user that would effectively mean “please try again”.
Now, of course, I’m going to toss a very large caveat into this by saying there may well be another issue I’m not aware of, and that the problems could occur much more frequently than I’m guessing they would - in which case more care must be taken to prevent or resolve these issues.