I have tried to find where I read it but there was a post on a chat room which said that is was not necessary to earth or bond the waveguide to the source (Apparently the writer epoxyed the horn onto his magnatron/chassis?)
Not sure why it could not be attached with bolts/screws as usual but it did raise the question in my mind as to the theory. Does a bonded waveguide present a better path or does it not matter!!???????????;-)