The point is that if the transformation that Stuart uses were applied to a single agent, it would convert a coherent utility function into an incoherent one; therefore it cannot demonstrate anything about the incoherence of combined utility functions. It is too general - in fact, it is a Fully General Counterargument to the existence of utility functions with more than one input. It could well be the case that independent agents cannot have a coherent combined utility function, but this argument does not demonstrate it unless you also wish to assert that single-agent utility functions cannot consist of linear additions of sub-utilities.