Dave Urie, Have Blue

I believe the writer is using an alternate concept or is not
communicating effectively. The term "aerospike" refers to the use of a
vehicle's shock wave as part of the thrust structure of the propulsion
system. The plug nozzle engines (both linear and annular) are aerospike
engines, but they are not the only kind. The sort that has been
commonly associated with the rumored "Aurora" involves a hypersonics
design in which the airframe has a defined crease point across some mid
section, which holds a shock wave. Fuel is injected into the exterior
airstream at this crease point and undergoes either continuous
combustion, or pulse detonation, using the rear fuselage and the shock
wave as the thrust structure. Vanes might be used to control airflow
and to serve as fuel injectors into this airstream.
That being said there is an alternative explaination: while linear and
annular plug nozzle engines do not breath air, they can derive
additional thrust from the rear fuselage shock just as described above
if the thrusters are operated in a fuel rich mode. They also generate
thrust against the bottom plate of the nozzle, particularly if
turbopump exhaust gases are outlet in the plate region to counteract
the normal tendency to develop back flow vacuum. Note that the X-33
developed some rather large air dams around its aerospike engines to
both prevent air from leaking in the sides into the plate region or
into the thrust ramps, as well as cross spillage between thruster
sections in order to effectively thrust vector. The X-33 also required
some rather large body flaps to control rear airflow. Either the air
dams or the flaps, or both, may be what the writer was speaking of.
There is also a third explaination: that the aerospike engines were
augmented by a ram-ejector cowling to improve Isp and thrust in
fuel-rich mode. What advantage that would provide over the rear
shockwave is unknown.