If you were correct, and state of assembly / separability wasn't an issue, then why wouldn't all firearms parts be considered firearms?
My bet is it has to do with what does disassembled mean . One can argue that simply having the mag out is disassembled . Just having the grips off . You can put the slide on a glock and most semi autos in a second and be ready to go . I can have my AR in two pieces and have it together and shooting before most could load and fire a revolver . Disassembled does not really mean inoperable .