This is a cool video, which I have not seen before. Thanks, for posting it. I remember on the old 4-segment Shuttle SRB these plugs were made of styrofoam. I imagine that is still the case. To some extent I imagine the nozzle plug (The plug is in the divergent nozzle exit cone. It may be held in place by some weak adhesive.) was a hold over from the idea that some plug was needed to burn up pressure in the chamber to the get burn-rate up rapidly. (Even Brinley talked about this in his Amateur Rocketry Handbook nearly six decades ago.) In all likelihood the plug acts more as a weather seal. The plug is not necessary for the SRB ignition. Somewhere out there is a video of a Shuttle SRB static test with no nozzle plug and a camera looking down the bore of the motor catching the entire ignition event. It is a cool video. I remember seeing the video years ago. You can see the flame spreading in the forward propellant fins. The ignitor has choked flow with high under expansion, which means that the exhaust expands like crazy outside of the throat (there is no divergent nozzle).