(their probaly WERE some people as low rank as lieutenant commanding defiant class ships given how small those are.
They are small, but they are new, and they are pure warships, with the capabilities of a much larger ship. You wouldn't want anyone under Captain in one of those.
I could certainly see a Lt. captaining one of those century-old Miranda-class ships, though, cataloging gaseous anomalies and the like. Probably wouldn't be the ones going into battle, though (but that would explain how they were meat shields