It sounds more like a muffler design than tuned pipe design. I made some experimental mufflers at one time, and they also utilized baffles and inner piping. They were very effective at reducing the decibel level of the exhaust without undue power loss. But increasing power they did not.

The reason for the baffles and pipes inside is to change the direction, and to expand and compress, the exhaust gases. IIRC from a book I read about 20 years ago, every time you change the direction 180° you get a reduction in sound pressure level. IIRC the expansion and compression is for keeping up the velocity of the gas.