Unlike a twin shock Type247 where the sub-frame contributes to the structural integrity of the chassis ... or ... (laughing) ... it's SUPPOSED to contribute ...

the sub-frame of the X-Frame simply provides a platform to hang a seat base, the rider, and some electrics stuffed under the seat-hump. Even with this limited task, the structure must be solid in order that the rider get optimum and REAL feedback from the chassis:

The light weight but sturdy aluminum sub-frame takes shape:

A detail of the connections used to bolt the sub-frame to the main: