This relies in willfully mistaking figurative language for non-figurative speech.
Roosevelt didn't "trade" those people in the sense that a slave trader traded his captives. He made a political pact which included handing prisoners of war to a foreign power. There was no selling of human individuals in that; it is a "trade" only rhetorically, in that a political pact involves both sides making concessions, which may be roughly similar to the concessions made by people selling and buying things.
Also, those people were not "slaves" in any technical terms; there were no "slaves" in the Soviet Union. Slaves are property that can be bought and sold; we use the word "slave" in many different figurative senses, some very foolish, others not so much. And the condition of POWs returned to the SU may have been dire, even similar to slavery in the 19th century Americas, so the rhetorics may have some merit here. But to attempt to make a factual statement that "Roosevelt was the biggest slave trader in recorded history" and intend it to be taken as an accurate description of what really happened is to mistake electoral campaign rhetorics for a scientific description.