So, first off: Yes, that is true. The source he mentions is Anne Applebaum, a known scholar of the Soviet Union mentions this phenomenon several times in her book: Gulag. A History. (New York 2003) on pages 137-40 and 143-45.

Applebaum writes:

Why the Soviet Secret police were so obsessed with confessions remains a matter of debate, and a wide variety of explanations have been proferred [sic] in the past.

(page 139)

She herself goes on to list a number of possible explanations, including Stalin's neurotic obsession with making others confess what he himself had done (he had been an informant for the Tsarist police at some point), that Stalin wanted to destroy his opponents physically as well as morally or that it was an important part of the procedure for the NKVD officers themselves giving the whole procedure an air of legality as well as legitimacy because -- as Žižek explains -- it gives the illusion that the person being arrested and interrogated is still redeemable not just to the investigator but also to the convict.

This phenomenon of the confessions and especially of those caught in the Stalinist purges who themselves freely confessed to counter-revolutionary activity despite having never taken part in any such thing, is a phenomenon that has interested quite a few people researching Stalinism. Hannah Arendt e.g. in her book The Origins of Totalitarianism has also grappled with this vis a vis the Nazi system. Arendt uses it as evidence for the totalitarian nature of Stalinist rule. According to her, Stalin's totalitarian rule was so total that it basically convinced people to testify against themselves. But what has even Arendt stumped -- she refers to more research being needed -- is that the Nazi system which she places in the same category of Totalitarianism, this phenomenon is conspicuously absent -- as are purges of the movement in general with the exception of the so-called "Night of the Long Knives" in 1934.

As a scholar of Nazism, this phenomenon of confessions and sometimes voluntary confessions is indicative for important differences between the regimes, and this practice is the result of very different conceptions of who is the enemy and accordingly a different style of rule.

In the conception of their enemy »the Jew«, the Nazis constructed the ultimate »other«. »The Jew« was everything the member of the Aryan race and the German Volksgemeinschaft (roughly translated, the racial or people's community) was not -- and even more importantly, he could never be because of his racial characteristics. In the Nazi conception in its form as an ideal type, there simply was no Jew who even had the potential of being like a member of the German race. He could only pretend to be in order to fit in and use this deception for his evil machinations.

The same is true for almost all other political enemies of Nazism. Asocials had spurred the Volksgemeinscahft and through their behavior kicked out of it, communists and socialists were pawns of the Jews, Slavs the same but in general being born to be subjected by the German race etc. The race and Volksgemeinschaft aspect of the Nazis' conception of their enemies has a finality to it that is projected outward of the community, even for the Asocials since through their behavior they forewent their membership of the community. This is also the reason why according to the Nazis every Jew had to die. There simply was no chance to be redeemed because of their racial character (side note: This does not apply to all persecuted by the Nazis equally. There were German social democrats let go from Concentration Camps in the 30s and Witnesses of Jehovah could if they gave up their fate be released, however, the war strengthened the community aspect since in a struggle between life and death, everyone working against the community was such an immense threat that they had to disappear).

Stalinism on the other hand lacked this totality of the race aspect in their conception of the enemy. Stalinism at least when it came to the purges projected the enemy inward and made out those it persecuted not so much as enemies directed from an outside force but as enemies driven by a different interpretation of Marxism, Trotskyites to be exact. Yes, there were those who confessed to being imperial agents but the "counter-revolutionaries" in a majority of the cases confessed to being Trotskyites or other adherents of certain interpretations other than Stalinism, LeftComs and so on. Heterogenity and those who in the minds of their torturers and jailers worked against the socialist movement from within were the most dangerous enemies.

Also, the Stalinist purges were directed towards people in higher positions in the party and the military rather than groups who had been perceived as social outsiders for quite some time like Jews (again, side note: There also was persecution of social outsiders and people made social outsiders in the Stalinist system but the insistence on the confessions seems to have been something more directed toward those who had occupied positions of power within party, state, and military).

Both conceptions of the enemy relied on a certain set of paranoid ideas but those were channeled towards different centers of gravitation if you will: Stalinist afraid of their own tools of power while the Germans constructed an external thread.

Sources aside those mentioned:

• Moshe Lewin: The Soviet Century.

• Moshe Lewin and Ian Kershaw: Stalinism and Nazism: Dictatorships in Comparison.

• Ian Kershaw: Hitler, The Germans, and the Final Solution.

• Geyer, Michael; Sheila Fitzpatrick: Beyond totalitarianism: Stalinism and Nazism compared.