How is it poisoning the well?
You're kidding right? Did you not read the text you quoted?
Let's take a look:
'Because its goal proved so elusive' = poisoning the well by priming the reader/viewer into thinking socialism can't/hasn't achieved its goals.
'the socialist movement split and split again into diverse, sometimes murderously contradictory forms.' = Murderously contradictory? Again, primes the reader/viewer, this time with colorful language. Socialism is contradictory! Socialism is violent!
'There was Social Democracy, which insisted that only peaceful and democratic means could produce a harmonious commonwealth. There was Communism, which extolled the resolute use of force and dictatorship to propel mankind to a new way of life. There was Arab Socialism, African Socialism, and other Third World variants that sought to amalgamate western Social Democracy and eastern Communism. There was even fascism, which turned the socialist idea on its head by substituting the brotherhood of nation and race for the brotherhood of class. And there were those - from early American settlers, to the "flower children" of the 1960s, to Israeli Zionist kibbutzniks - who built their own socialist communities, hoping to transform the world by the force of example.' = primes the reader/viewer into thinking of socialism as hopelessly fragmented and chaotic, and does so by dishonestly conflating systems and idealogies that actually have very little to nothing to do with each other.
'As a model for the development of post-colonial states, the socialist model proved disappointing, fostering economic stagnation among millions of the world's poorest people.' = blames a priori the economic troubles of these states on socialism (and not, you know, the same reasons that keep many post-colonial capitalist states in similar poverty), also poisoning the well by insinuating that it would do similar things in developed economies that can actually sustain socialist policies, even if they're not outright implying it, they should be more than capable of understanding that less skeptical minds will innately jump to that subconscious conclusion.
'And in its most violent forms, socialism was calamitous, claiming scores of millions of lives and helping to make the twentieth century the bloodiest ever.' = poisons the well by conflating socialism and authoritarian communist states; priming the reader/viewer into thinking of socialism as an idealogy that leads to violence.
'Through profiles of the individuals that brought socialism to life, HEAVEN ON EARTH tells the story of how an idea arose, evolved, changed the world, and eventually fell.' = poisons the well by saying that socialism 'eventually fell' (neither the idea of socialism not socialism in practice has fallen); thereby insinuating that it can not stand on its own and socialism is doomed to failure: ideas that have risen and fallen are things we consider to belong in the past; so by referring to and idea that is still very much alive and practiced today in such a way, they're poisoning the well by priming the reader/viewer into regarding it as an outdated concept.
It would be one thing if they made these arguments in the documentary itself; their arguments would still be wrong but they wouldn't be poisoning the well at that point. However, stating these things beforehand is even more problematic. An objective teacher (as documentaries should aspire to be) should never put the conclusion before the facts. You don't start out by saying 'X sucks, here's why.', even if you hide it in more subtle language.