That kind of graphics tablet was just an input device, typically used for digitizing drawings which had been either drawn by hand or generated by some kind of device without a computer interface.
It has nothing to do with tablets in the sense of small computers.
Its design was mostly a hardware design problem, somewhat more difficult than designing a mouse, because it had accuracy requirements, but not much more.
It's weird that you need to explain this to people who've never used a pen tablet without a screen. I mean, many of us have used wacom pads as standard input devices for decades. With a bit of hand-eye coordination, it's amazing what humans can do.
It has nothing to do with tablets in the sense of small computers.
Its design was mostly a hardware design problem, somewhat more difficult than designing a mouse, because it had accuracy requirements, but not much more.