[Si Coe]

Additionally as I understand it the TRF cars aren't actually made at the main Tamiya factory but produced by a small separate company for Tamiya. Which means they can use fundamentally different technologies.

In any case, the materials used differ between applications.
The DF03 though race worthy is primarily aimed at the basher market, and was never originally intended to house monster brushless systems. It was built to be reliable with minimal maintenance using basic 540 class motors down to no lower than 19t brushed.
The TRF cars are no nonsense racers. They need more looking after with exposed drivetrains etc, and use stiffer plastics that reduce flex to aid setup, but are more fragile in a high speed collision (DF03 wishbones bend and pop out, but don't break). They were built for massive horsepower, but at the trade off of more servicing.