Technically speaking it is route specific as you say but it doesn't help matters. Take Southampton to Brighton route 'Not London', for instance - there are at least two valid routes for that, of which neither share a common station other than the origin and destination.
Using such a description to help determine what could be a valid route (without assistance from the NRG) is useless and gets you nowhere. When there is also a route 'Barnham' available then you get the idea you should go via Barnham (i.e. only one such route).
If we look at things in South London, picking something random out of a hat - East Croydon to Wimbledon. Only route available here is 'Not London' but that doesn't help the passenger make a conscious decision about a valid route - 'Not Clapham Jn' would make things clearer for a passenger (e.g. Earlsfield to Crystal Palace).
There are many possible routeings for such a journey which are 'Not London':
1) Tramlink
2) via Clapham Junction
3) via West Croydon and Sutton
4) via (West Croydon,) Norwood Junction, Crystal Palace, and Tulse Hill.
Judging by the price of the ticket, I conclude the route expected is option 2), in which case they should make a positive route description (of 'Clapham Junction') rather than a negative (of 'Not London'), to assist the passenger.
As I said, based on your quote of the NRG
It can be used on any route not listed in the Routeing Guide for which a lower priced route specific fare exists.
Well the route I suggested above for East Croydon to Ebbsfleet isn't listed, the route specific 'Not London' is cheaper, undefined in the routeing guide, avoids London, and does not involve doubling back anywhere. By the instructions that means I can choose any route in the guide that doesn't include London. So let's go via Birmingham instead - that isn't London... The sense of unreasonable-ness here says that it can't be valid.
That's not to say there won't be people who don't take the mickey - obvious answer would be to change 'Not London' into something that is more meaningful, and gives an idea where travel should be via. After all, I can only suspect such a ticket is to be used via Herne Hill or Peckham Rye but have no definitive answer.
'Not London' is already odd enough as it is meaning 'do not touch a London Terminal' (or at least that's all I can assume, judging there is no definitive answer to what it is*!). London is a big place, and to travel via Clapham Junction (in zone 2, London) on a 'Not London' ticket seems very unintuitive but is valid. Having 'Not London' fares for flows solely within the zones is something odd too.
*The likes of 'Not Clapham Jn' or 'Not Doncaster' are slightly more specific, but could be argued to be as meaningless as 'Not London'. The fact that a 'Not London' can be used via London if it is more expensive than the route 'London' is also rather silly...