I don't believe that there was ever extra chips being added to the boards, but what I could believe is that they shipped with firmware on specific chips that enabled data exfiltration for specific customers and due to a game of telephone with non technical people it turned into "they're adding chips inside the pcb layers!"
I thought the point was an extra chip in the place of a pull up resistor or something that would edit the firmware image as it made its way across the bus, so you wouldn't see the modifications even if you pulled the flash chip and read it out manually, and would also be persistent across flash updates.