I don't think we saw a hard distinction until the time of Domitian, when the Roman emperors finalized the transition from the fictional "first among equals" to semi-divine figures ruling with undisguised autocratic powers.
Augustus was technically just "first citizen", and you could pretend that Rome was still a Republic for quite a long time.
I think there's room for a lot of takes, but I know what my preferred theory is. Obviously all of this is non-canon.
Noble savages. The Empire lives without the harmonious wisdom of the Emperor, so they can only be barbarians. However, they are far away from Cathay, and the only contact Cathayans have with Imperials comes from diplomats and merchants, who are very polite to their hosts. So they're good barbarians, the sort of people who have all kinds of noble virtues.
Cathayan books on the Empire tend to focus on honorable barbarian heroes who live in a frozen waste constantly besieged by malevolent spiky barbarians. They imagine Imperial nobles ruling from great halls, toasting each other with mead before they ride out to fight the Ruinous Powers. While these figures are unquestionably barbarians with all manner of strange customs and superstitions, they are quite romantic. Some authors, particularly those who live in cities, will emphasize the primitive virtues of a rough, proud frontier people.
Several very popular works of fiction feature a Cathayan hero/ine journeying to these barbaric lands, befriending the inhabitants, and defeating some scheme of the Ruinous Powers. They may or may not find true love along the way, depending on the genre.