Looking at the documentation motion_detected says
Returns True if the device is currently active and False otherwise.
which probably not what you are looking for.
there is an example on that page that goes like
from gpiozero import MotionSensor
pir = MotionSensor(4)
which make more sense
My guess is the sensor is too sensitive, picking up noise as indicated in the page you linked
On the PIR module you should see two orange components with sockets that fit a Phillips screwdriver (see above). These are called potentiometers: they allow you to adjust the sensitivity of the sensor and the detection time. You should begin by setting the sensitivity to max and the time to min, but you can vary this later if you wish.