Normally with wiring your description of solid vs stranded wiring is correct.
Spark plug wires are in two general designs: 1)solid 2)resistor/suppression
The solid are as you have found. Stranded for flexibility as they need to move a bit. The solid refers more to the connection from one end to the other as the stranded wires are connected all the way through from end to end. Some versions over the years have indeed had a solid wire like you picture.
Resistive wires actually have a core made from carbon strands. The connection from end to end is actually poorer than the solid core wires as it was designed to reduce electro magnetic fields that were starting to interfere with radio reception. If you tried to use an old chunk of this plug wire to power a light bulb it would be dim since the wire has resistance and is not a "solid" connection like the solid core wires.
