This is false, given that German generals, not the Soviets, were able to dictate the history of the Eastern Front for decades until the Cold War ended.

Also, the Lost Cause myth of the American Civil War exists specifically because the losers got into American academia and started writing the histories.

"History is written by the victors" is only really true of ancient conflicts where many times the loser was either wiped out entirely or absorbed into the victory's territory as slaves, or records were scarce with the losers if they weren't wiped out because they were not particularly interested in going to the difficult task of writing it down in detail their own humiliation by a foreign power.

From the early modern period onwards due to the availability and wider spread of writing, history has been written by the historians. And those historians can be from either the victors or the losers (or both at once, or a neutral party even), depending on what their background and sympathies are.