Charlie,
I believe that you've reasonably expressed the concept of the instructors that the OP was reporting.
IMHO their line of reasoning is only partially correct.
For branch circuits, consider 210.19(A)(1) and 210.20(A).
For feeders, consider 215.2(A)(1) and 215.3
It is pretty clear that _both_ the conductors and the OCPD must have ratings of 125% continuous load + 100% non-continuous load. It is also pretty clear that this requirement is for the benefit of the OCPD devices, not the conductors. The Exceptions to the above rules involving the use of 100% rated OCPD are the basis of my reasoning. Conductors may be used at 100% continuous loading, as long as properly rated OCPD is used.
The line of reasoning being discussed here suggests that conductors must have ratings of 125% continuous + 100% non-continuous _prior_ to derating and adjusting for things like temperature and conduit fill, however after such derating and adjustment they need only have 100% rating for both. I agree with this interpretation of 210.19(A)(1) and 215.2(A)(1).
However 210.20(A) and 215.3 still apply, which means that the OCPD must have a rating of 125% continuous plus 100% non-continuous. This means that all of the requirements of article 240 are in play, and the conductors with their adjusted and derated capacity must still be properly protected by the OCPD.
In the particular example given, you end up with conductors of 180A ampacity feeding a 180A load, on a 200A breaker. This is entirely kosher because of 240.4(B) and 240.6(A).
But consider instead this same panelboard, but rather than 100A non-continuous plus 80A continuous loading, give it 160A of continuous loading, again high harmonic single phase loads where the neutral must be counted as a current carrying conductor. And lets tweak things a bit by putting the feeder into a 90F environment.
215.2(A)(1) that the conductors have an ampacity not less than that required to serve the load, and in addition requires the use of 200A conductors prior to derating and adjustment, that being 125% * 160A. For example, 3/0 THHN. The derated and adjusted ampacity of the 3/0 THHN conductors is 225 (300.16) * 0.94 (310.16) * 0.8 (310.15(B)(2)(a) ) = 169A, which is sufficient for the load. So (by the current reasoning) we are good with both parts of 215.2(A)(1).
However 215.3 still requires the use of 200A OCPD.
But 169A conductors are _not_ properly protected by a 200A OCPD, even though they are sufficient for the load served, and meet the requirements of 215.2(A)(1).
-Jon