I think the question of overlap isn't so relevant as is the question of activity.
I'm sure we all can agree that just about any StackExchange-suitable question there is about Tor is currently in-scope of an existing StackExchange site - most likely one of SOFU or IT Security. However every now and then a subculture grows, within a site or group of sites, which has such a niche interest group, and yet such high volume, that it warrants spawning off a new site of its own. Questions on this new site definitely would be on-topic for one or many other sites, but community built by the new site has a depth of expertise on its specific topic which might otherwise be lost in the noise of a community with broader focus.
This is how we've come to have sites like Unix & Linux, Ask Different, IT Security, etc. Nearly every question on those sites is a suitable question for one of the original trilogy sites, and yet they've spun off thriving and successful communities of their own.
So for something like this, where questions aren't just possibly on-topic for existing sites but already being asked and answered, the deciding factor doesn't so much need to be about whether it has a unique scope that doesn't fit anywhere else. What is relevant though, is to ask whether or not the existing tor questions across all sites in the network are being asked and answered in such a volume as to support a whole new site.
I don't have access to all the stats myself, but my guess would be probably not.
So, I just took a look across several sites for questions tagged tor.
- IT Security: 53
- SuperUser: 41
- StackOverFlow: 189
- Unix & Linux: 14
- ServerFault: 18
- Ubuntu: 70
- Apple: 1
So, that's 386 questions with the oldest dating back to 2008 - or about 78 questions per year. Hardly indicative of a self-sustaining community.