The criticism is that you're not up to date on the current state of your own field of specialty, and it is not a light matter. If the "reviewer" is some sort of committee member that can throw a wrench into your exit process, you should address the criticism.
The real issue is how to address it. My assumption here is that your defense will be "typical", with a formal presentation followed by some sort of closed session.
You have a very limited amount of time before your defense, and you need to use it wisely. To start, I'd pick the five most relevant citations to your work that you have, and search them forward to see who cited them, and read those papers-- there will probably be less than 20. If nothing earth-shattering has happened, you will eventually add those to your reference list, and all should be good. If there is something earth shattering and pertinent to your work, you probably have more research to do at this point.
How will your reviewer know you did it? Well, for the closed session, you should have a slide in your rack entitled "review of recent literature". In closed session, make the opportunity to say "in review, a criticism of how up to date my knowledge of the field was came up. This was accurate, and this is how I've responded... I think I've covered this, and will continue to stay up to date with the literature".
To add, they may require a bit of a rewrite-- that's actually fairly common-- but worry about that when the time comes. The strategy is to to convince them that you've done the work, and you're ready to do that rewrite. You don't have the time to do it between now and your defense, and my experience is that the document is largely sealed until it's defended anyway. You're trying to set things up so your revised dissertation will be signed off on without a thorough going-through.