The NEC typically uses the word 'conduit' as part of a name like 'rigid metal conduit', or 'flexible metal conduit', and not as a general term. There is no definition for 'conduit' in article 100.
The use of the word 'conduit' in 310.15(B)(2)(c) may have been a mistake, it would be more of the NEC style to use the word 'raceway' in this instance. I would like to think if there were a cable installed instead of a raceway that it would be subject to the same temperature and therefore conductors from any wiring method should be included in 310.15(B)(2)(c).
Raceway.
An enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables, or busbars, with additional functions as permitted in this Code. Raceways include, but are not limited to, rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquidtight flexible conduit, flexible metallic tubing, flexible metal conduit, electrical nonmetallic tubing, electrical metallic tubing, underfloor raceways, cellular concrete floor raceways, cellular metal floor raceways, surface raceways, wireways, and busways.
Perhaps the available data, based on the tests, was only for conduits, so extending it to all raceways would not have been supported by data.