The code never says "just breakers", it talks about ALL overcurrent protective devices.
Simple question does the NEC allow loading a typical circuit breaker 100% for more than 3 hours?
No.
all fuses and breakers are tested by UL at their 100% rating. The "3hour derating" comes in when the protective devices are mounted into enclosure.
That is not what the UL 2004 general directory says.
CIRCUIT BREAKERS, MOLDED-CASE AND
CIRCUIT BREAKER ENCLOSURES (DIVQ)
Unless otherwise marked, circuit breakers should not be loaded to
exceed 80 percent of their current rating, where in normal operation the
load will continue for three hours or more.
The area UL brings the enclosure into question has to do with the 60 or 75 C rating.