Looks like a "servo saver". Not really necessary but some prefer to use them to absorb combustion impulse shock and prevent it from reaching the servo. The pulse vibration occurs too fast for the servo saver to react effectively.

I use the carbon rods as well for just about all my throttle installations. Rigid and light. One of the advantages of flying larger planes is you get to position the throttle servo just about anywhere you want it, and straight shots with the linkage are generally the norm. The old glow installations usually had the throttle servo farther away or tucked into locations that required serpentine routing to get to the engine.