So in that case there is no materials secret sauce guarded by TSMC, with the customer either submitting the GDS file with all the info, or signing off on the modified version.
Sorry my comment wasn't very clear about this. There's still secret sauce in the materials. The GDS is purely dimensional. It's just a bunch of shapes. But how those shapes get translated into actual silicon, and how various chemicals are used and in what concentrations to manipulate the silicon to make the transistors function is the secret sauce.