Hey Philip
app.executeCommand is an unsupported function, or a "private api" as Apple calls them so it's best to avoid using it as there is no guarantee that it will work. I use it only when there is not other option. In the case of New Comp from selection, it is very easy to do that through standard api's:
Code:
var projSelection = app.project.activeItem; //the selected item in the project window
var myComp = app.project.items.addComp(projSelection.name, projSelection.width, projSelection.height, projSelection.pixelAspect, projSelection.duration, projSelection.frameRate); //Create a new comp with the same dimensions as the selected item
myComp.layers.add(projSelection); //add the item to the newly created comp
-Lloyd