Well, you can't.

You can buy a wacom tablet with tilt sensors and make the robot replicate the same stroke. You could add randomness to it too.

To apply different amount of force with the robot you use a spring and you control the force with the distance to the paper, it is very simple to do.

We did exactly that with ABB robots in the University a long time ago.

We used wacom drivers for linux and the wacoms were over 200 dollars or so.

A robot that uses only 3 axis is not "five figures", it is basically a plotter and thanks to reprap you could make your own for over 300 dollars.

Those are probably the ones used by this people, but you can use a robot with more degree axis and you won't be able to differentiate it.