Circuit breaker ratings is a 100% design rating, other factors such as heat build up can lower that rating. So yes I can carry the rated load unless heat builds up to point that it lowers the setpoint. I deal with this at where I work, where circuit breakers can trip below rated load and at other times will carry its rated capacity even slightly higher for hours if not all day due to temperature differences. Since I am no where near the rated capacity of the system configuration I am not too worried about that, at least yet. The .8 factor is just a common accepted design margin to size the circuit to allow for other factors, does not mean the circuit cannot handle the full rated load.
https://www.ecmweb.com/basics/sizing-circuit-breaker
The .8 "factor" is NEC in regards to continuous duty.
I deal with this at work as well, being an industrial maintenance electrician.