The single OBJ file has multiple individual objects, neatly aligned so you get a complete assembled unit when you view them all at once - but they’re still individual objects.
The object you’re looking for is labelled “object_82” in the file.
EDIT:
In case whatever tool you’re using doesn’t like handling multiple objects, I loaded the combined OBJ into Blender and exported just the back panel for you. You can download it here for the next 30 days.