Yes, That is true - he must cut open the current gasket, through the seal, to take the windshield off - clean out the edges and glue-on the new windshield w/ the gasket in place. Windshield is basically hold in place by glue, and the plastic gasket only provides support and covers the gap between the frame and the glass.
I would go with some one who has done this before and has experience with BMWs. It should not take more than one hour to do this, and takes 24 hours for the glue to dry out before removing the tapes.
Rgrds,
/A