That is only the case if you consider DMOZ to be some sort of listing service - its not... as such the concept of a "backlog" does not really exist.
There is only a "backlog" at DMOZ if you consider DMOZ to be a listing service and the editors role being to process submissions - it is neither. Most critics and that author consider DMOZ to be a listing service ...they need to get over that. Many other directories provide that service. I fail to understand why so many complain about something that is not provided by DMOZ.
An editors role is to build a category of useful resources. The role is not to list every possible site on the topic of the category and its not to process submissions. Submitting a site is nothing more than a suggestion for the editor to consider (and editors are free to totally ignore all submissions, though most do not.)
To build a category an editor uses the following resources (for eg):
1. Personal knowledge
2. Following links on sites already listed and not listed
3. Searching Google and Yahoo, etc
4. Google alerts
5. Print advertisments; signs on trucks etc
6. Submmitted sites
7. Industry publications
By submitting sites, you are doing nothing more than providing another means of assistance for a editor to find good sites. The problem is that the submitted sites is the worst source of good sites and is the most inefficient way for an editor to build a category. The best sites are probably never submitted (not every one has heard of DMOZ!) - they have just as much right to be considered for a listing to build a category of good resources as any site submitted. As they were not submitted, they are just a bit harder to find.
From this point of view the "backlog" at DMOZ is an irrelavant concept. The "backlog" really consists of the entire www of sites not listed.
DMOZ is NOT a listing service for webmasters. That servce is provided by a lot of other directories