no way they could hide so many, even USSR in era without Internet would have troubles hiding so many casualties (widely dispersed)
I disagree. And the linked article describes clearly how Putin accomplishes this.
Russian president Vladimir Putin has decreed that all Russian casualties “in peacetime” be a state secret. In addition to criminal charges arising from divulging state secrets, families risk losing pensions and lump-sum payments if they reveal that their sons were killed in Ukraine. Mothers of soldiers’ associations have been branded “foreign agents” for collecting data on Russian casualties. Dissident Boris Nemtsov was murdered shortly before completing his study of Russian casualties in Ukraine. Russian civil rights organizations, working against the fog of official resistance, could confirm only several hundred battlefield deaths.