I do not believe that is the case. I know Richards chassis manufacture in the way you have described using four flat pieces welded together at each corners, but I believe that Marsland chassis manufacture using the c-sections the same as land rover did originally.
That’s because Marsland don’t manufacture the chassis - they buy them from GKN who also made them for Land Rover. Hence why, apart from modified engine mounts etc., the chassis are to the original spec.
