CNN’s Cold War documentary repeatedly states that communism seemed appealing to many in impoverished Europe after WWII. Why?

At the end of the Second World War, Communism and Stalin did not appear to many in Europe as being synonymous with totalitarianism. Truman, as you quote him, meant to persuade Congress to support the containment of Communism, and so of course Communism was put in the worst light as possible. But Communism had a far more complicated reputation at the end of the war, and this reputation was positive for several reasons.

First, "democracy" as a concept had been largely discredited during the interwar years. Liberal democracies had failed to prevent the Great Depression with many of them falling to Fascists governments. The celebration of democracy at the end of the First World War seemed silly in retrospect, and many no longer had faith in the strength of democracies or in their inevitability. In contrast, the economy of the Soviet Union had not collapsed during the Great Depression.

Moreover, Communism had not been discredited by the rise of Fascism. The great liberal democracies of Europe, France and Great Britain, had sold out the smaller democracy Czechoslovakia in 1938 at Munich in order to protect their own interests. The Soviet Union, however, had opposed the German annexation of the ethnically-German area of Czechoslovakia, and so Stalin remained inculpable at Munich. True, the Soviet Union then sided with Germany for a time and invaded Poland under this alliance, but the Soviet Union could and did portray itself as having no choice but to side with Hitler after the West had refused to take a stand against him. And the long war, which the Soviet Union won against Germany, absolved the Soviet Union of its guilt.

And, of course, this point is most important of all: the Soviet Union, and by extension its socialist system, had soundly defeated the Nazis and liberated tens of millions of those people you cite who were impoverished by the war. At the end of the war, the overwhelming opinion throughout Europe was that the Soviet Union deserved most of the credit for Germany's defeat, and for good reason: the Soviet-German front was the largest of the war, and the Soviets were responsible for 9 of every 10 German causalities over the course of the war. The Red Army stormed across Europe into Berlin, liberating millions on its way. Thus while France and Britain, and by extension the weaknesses of their liberal democratic systems, appeared responsible for the Second World War (their appeasement of Hitler at Munich), the Soviet Union seemed the global power that ended it.

Things would seem very different a decade later, but at the end of the war, the Soviet Union seemed to many a beacon of hope (the Czech Communist Zdenek Mlynar, who joined the Party at the end of the war as a teenager, literally calls the USSR a "beacon of hope" to describe the Soviet Union at this time). Communism, after all, offered an alternative to both liberal democracy and Fascism, two systems that had (seemingly) failed millions. And Communist rhetoric only reinforced this perception. Marxism-Leninism, after all, predicted that the Western "bourgeois" states would continuously plunge Europe into war as they reached their final stage. Having just survived the war, many were receptive to the Soviet message that future wars could be averted and that a new social system could emerge that would liberate the masses from the horrors of capitalism. The idea that the world could rise from the war better than before appealed to those who wanted a clean break with the past.

And, finally, as Truman argued, impoverished people tend to look for any new leader or ideology to save them from their plight. Understandably, Truman ignored the nuances of Communism's appeal when making the case for exerting American power to "contain" it. His anti-Communist stance soon appeared reasonable anyway, as many of these nuances that had made Communist popular evaporated in the coming years. The Soviet Union clamped down on its "sphere of influence" with the use of secret police and sometimes even Eastern armies, curbed the liberties of the people within its empire, and proved unable to organize an international socialist economy that could live up to the Soviet Union's postwar promises.

Some very readable sources:

Mark Mazower, Dark Continent: Europe's Twentieth Century (2000).

Bradley Adams, The Struggle for the Soul of the Nation: Czech Culture and the Rise of Communism (2005).

Zdenek Mlynar, Nightfrost in Prague: The End of Human Socialism (1980). {This memoir spells out in detail in the first dozen or so pages why Communism appealed so strongly to Mlynar as a teenager who had just lived through a war that started in part because Britain and France had reneged on France's military alliance with Czechoslovakia by conceding to Hitler that he could annex a portion of Czechoslovakia}