Incorrect
In the rulings of the gully type tables, when a ball has left the table surface and is in motion within the gully and has no opportunity to return to the table surface, it is considered pocketed.Very good.
And thus, when a ball is pocketed, and it slips through the gutter and onto the floor, this is a foul. As the deciding factor is the resting place of the ball, and if the resting place is the floor, it is by rule a foul.
And, if the ball is pocketed, and slips out of the ball return and onto the floor, this is a foul as well. We simply use the final resting spot of the ball as the defining factor.