"Nominal" is usually considered synonymous with under load. It will be in regulation, but how long that remains true depends on a variety of factors. Keep in mind that if it were DD, the emitter would be getting closer to an amp, possibly higher(?), of current-more than it does in regulation, and current draw from the battery would be somewhere in the neighborhood of 900mA...it should be about 3.6 Vf. That is also the nominal voltage of a single RCR123. Put the cell under load, and the voltage will depress even lower.
Basically, as long as the current and output seen at the emitter would be greater in direct drive than the level designated by the buck circuit, the circuit should be able to operate in regulation. That is not 100% correct, because it doesn't take things like temperature and driver efficiency into account, but might serve as a simplistic way of illustrating how a single Li-ion can work with a buck circuit.