Developer OK for hardware/accessories but it is complicated as knowing who is and isn't the developer. Sometimes the developer is not the manufacturer or whoever's brand name is on an item. An example of this can be found in accessories where "someone" develops the hardware but is then private branded or has a custom shell and sold under the name of many different companies. Sometimes an actual developer of hardware/accessory is a person.
In general I'd recommend to not set developer on this type of entry but the Style Guide is correct as far as Yes for Developer and No for Publisher.
Hardware isn't "published" so there should not be publisher set. We already have a bug report regarding the situation where Developer or Publisher cannot be blanked out or set to default like you could with say... rating or the other drop-downs.
This is but one of many situations where the item entry layout, being designed to track "games" does not match up to the type of information you'd want to have for hardware. Same thing with books or toys or movies or whatever else we have entries for that aren't games.