It's to highlight that they know Pu Yi is a mere puppet "emperor" of "Manchukuo" (which in turn is a mere Japanese colony). They treat and disrespect him as the puppet that he is.
Lines by Pu Yi before the one you quoted:
When the two Emperors stood together and saluted the two national flags, they recognised that Manchukuo had come of age. Manchukuo is not a colony, Manchukuo is Manchuria. The relationship between our two countries is like the relationship between its two Emperors.
Their leaving is juxtaposed with the above lines (to highlight that everything Pu Yi is saying is nonsense).