The world consists of a number of competing civilisations. Western Europe is a Germanic-Roman civilisation which is, at present, subordinate to the Anglo-Saxon one. The latter is dominated by the US. Germany is fed up with this situation and lately has been attempting, unsuccessfully, to break free from the Anglo-Saxon chains.
The Roman empire died because of internal decay, when material benefits were placed at the top of empire’s development agenda. Similarly, the Union State between Russia and Belarus cannot exist with a sole purpose of raising the well-being of its citizens. It needs the unifying idea, and that is that Russia and Belarus are part of Russian civilization, the Russian World, which has fairness as one of its underlying principles. The Russian World is intended to gather other peoples of Northern Eurasia around the eastern Slavic core. Northern Eurasia peoples are those who were involved in building the Russian empire and the USSR.
The Russian World ('Russkiy Mir') concept is closely linked with an an imperial Russian and Russian irredentist ideology of "all-Russian nation", a recurring pro-Kremlin narrative aimed at weakening the national identity of Belarusians and Ukrainians as well as their sovereignty.
Earlier cases alleging that Belarusians are part of Russian people can be found here and here, whereas other examples of pro-Kremlin disinformation outlets' attempts to question the Ukrainian nation can be seen here and here.