Perceptual modes are specific mechanical stimuli we learned to recognize as specific sensations.
Some of them are: stiffness which is skin indentation and tendons strain in function of space, vibrations which is skin indentation in function of time for frequency above 10/20 hz.
These modes resides a bit on the neurophysiology of touch (how mechanoreceptors respond) and a bit on how we learned to recognize touch sensations.
The problem reside that the first thing you want to do is manipulate, not simply touch.
Manipulation is a bitch because it mostly needs small indentation of the skin which are 1-3 mm of continuous displacement at 1 mm resolution under the fingertip.
They piezo they are using displaces 1 mm at the resonance frequency, I imagine 150 hz (I believe?) or something similar, not in continuous (again, I believe).
Which means that a contact on the sensor is rendered as a local vibration, which requires a lot of mental gymnastic to think of it as a contact.
That is the haptic metaphor. Basically you expect an indentation and you get a vibration, and this sucks and for users.
If they got it in static displacement that is cool and might be something noteworthy