FontWeightNames are not a property of a document, but of the top-level app object. Try this:
I may have been confused by the documentation - I thought a "Session" was the open Book or Document:
"Denotes the font weight (specifies an index into the array of font weights provided by the session property
"Session" is actually the top-level-object in the FDK (Frame Developers Kit) and FrameScript. In ExtendScript, the "app" object is the equivalent. They probably used app because it is more in line with other Adobe scripting environments. The documentation was probably derived from the FDK docs and maybe that is how Session got in there.