It works for both.
matl_path = Sketchup.find_support_file 'Siding Rusty Metal.skm', 'Materials/Brick, Cladding and Siding'
#=> "C:/ProgramData/SketchUp/SketchUp 2018/SketchUp/Materials/Brick, Cladding and Siding/Siding Rusty Metal.skm"
matl_path = Sketchup.find_support_file 'Plywood.skm', 'Materials/Custom Wood'
#=> "C:/Users/Dan/AppData/Roaming/SketchUp/SketchUp 2018/SketchUp/Materials/Custom Wood/Plywood.skm"
Although I’m not sure which it checks first, the ProgramData or the User’s AppData paths.