What does the last sentence of Huaqing Palace mean that no one knows it is litchi?

The implied meaning is as follows:

The word "unknown" is thought-provoking here. In fact, "litchi lai" is not unknown, at least "concubine" knows, "riding it" knows, and there is another emperor who doesn't mention it in his poem. This writing is to show that this matter is very important and urgent, and there is no reason for outsiders not to know. This not only implies the absurdity that the emperor did everything he could to please his favorite concubine, but also echoes the unusual atmosphere rendered earlier.

works appreciation

Three Poems Crossing the Qing Palace is a set of poems by Du Mu, a writer in the Tang Dynasty. Selected Poems is one of them. Through the typical event of sending litchi, the extravagant life of Xuanzong and Yang Guifei was lashed out. It has a subtle artistic effect, exquisite and famous.

Poetry satirizes the present by borrowing from the past, and selects typical events and scenes such as offering litchi to Yang Guifei at great expense, living in a dream for a long time, An Lushan's behavior, and Yang Guifei's performance of dance, which not only skillfully summarizes the history, but also deeply satirizes the reality, expressing the poet's incomparable indignation at the supreme ruler's extravagance and indulgence in governing the country.

The whole poem is implicit and euphemistic, which means primate.