A few basics on Kopp's comments:
It is widely agreed (including by LockMart) that outer mold line is the principal driver of aircraft signature in the frequencies of greatest concern.
The most basic calculations of RCS are a product of Maxwell's equations and Ufimtsev's Theory of Physical Diffraction. Neither of these things are classified.
The limfac into the 1970s was the ability to model complex shapes and curves with reasonable confidence - that is, one would know that the model stuck on the pole would not have unexpected signature. The F-117 was made possible by working both ends of the problem: increasing computing capacity with hardware and software, and simplifying the calculation by using an all-planar shape.
Clearly, the computer hardware problem is no longer a limfac, nor does one need to see anything secret to write an RCS-estimation program or develop an accurate wireframe.
Materials are important but still have less impact on overall RCS than shaping, particularly at stealth-fighter levels. Mostly, what they do is take care of edges, apertures, holes and doors and other discontinuities. (If we could achieve all-round broadband - 30dbsm with RAM, we'd do it.)
LockMart and others have been careful to criticize Kopp for not having access to the classified details (no
, Sherlock) but have not said (a) that he's wrong or (b) that F-35 RCS is equivalent to F-22.
Subject to the Rice-Davies Stipulation, it should be noted that competitors have come to the same conclusions as Kopp, broadly speaking.