The inlet cones would certainly generate thrust but 80% seems optimistic.
The cones would slow down the intake air, ideally to subsonic speeds that the core turbojet could handle. Once hypersonic the cones retreat into the intakes and the shock wave from the back edge of the cone becomes trapped in the intakes and this generates very high pressures which are then bypassed around the core turbojet into the afterburner.
I guess they are talking about the pressure developed by trapping cone's shockwave inside the inlet as being equivalent to thrust?
OTOH I could be totally wrong.........