Yes, at face value, I read it the same way you are reading it. To LEO a 3U CubeSat (assuming 5 kg) would run you \$46k (\$8k*5*1.15) . This is significantly cheaper than say NanoRacks or Spaceflight rideshare services both of which are in the $250kish range for a 3U.
I would assume that the 15% includes the "overhead" related to rideshare, like the dispenser mass and other integration services.
Having said that - I would be careful in your budgeting. This is a vehicle that has not flown. Launch prices have a tendency to be significantly higher than originally predicted. And for rideshares basic economic theory indicates that Relativity would want to price rideshares as high as they can while remaining lower than the competition in order to maximize market share and profits. That would imply pricing at around $200k for a 3U. In either case, only time will tell what reality becomes.