Those other 29 tables do sort of count as decoration. But dwarves will still use them like unassigned tables with a dining room designated from them.
I've seen it happen. My dining rooms are arranged like this:
╔══┼┼══╗
║......║
║.╦╦╦╦.║
║.╤╤╤╤.║
║.ΩΩΩΩ.║
║.╤╤╤╤.║
║.╦╦╦╦.║
║......║
╚══┼┼══╝
The dwarves will use the closest table to wherever they picked up food, regardless of whether it was the table the room is designated from or not. This means that they're all given equal priority.