They state on their website that the "code features" which by the way look very conventional [0] are inaccessible to clay. As far as I can tell, sticking in a separate string of clay under each ridge would work just fine, then its just a matter of stitching it together within the standard design.
Funny how the article on The Economist states:
"A 3D-printed key that can’t be copied"
While their own claim is:
"A 3D-printed key that can’t be copied by the masses"
[0] https://youtu.be/RSVzsWMfTtc?t=52s