Not exactly what Dennis said. Your conductor ampacity can exceed the 75? C ampacity after derating. Sounds like he's not talking about the conductor.
I know it's not exactly what Dennis said, but it amounts to the same. And no, the way I stated it, my conductor ampacity cannot exceed the 75?C column's ampacity value. Dennis is talking about the conductor.
Let's use a 1/0 copper THHN for example. The table values are 150 and 170. If the derated ampacity is 140, then 140 < 150, so the in-use ampacity is 140. If the derated ampacity is 160, then 160 > 150, so the in-use ampacity is 150.