By inspecting the source code of the npm package, I understood what exactly is that inputString parameter. Essentially, every schema method returns a Type instance which has two properties: inputString & isPrimitive. What I tried (and worked out) is storing a struct definition in a separate variable and pass it to the array method like

glue.Schema.array(input_string=my_struct.input_string, is_primitive=my_struct.is_primitive