... which is still present to this day in some articles about his case, namely, that it's still possible that he was "Ivan the Terrible" of Treblinka, in addition to his service in Sobibor. The latter is not in doubt, as it is based on solid documentary evidence. But the former charge was based solely on eyewitness identification from the start, and was a pure travesty.
Contrary to Tom Teicholz, the eyewitness identification of a person 35-40 years after the fact is not trustworthy in principle. You just don't do that (and this is even aside from the fact that the identification procedures themselves in this case were found to be extremely flawed by Willem Wagenaar, one of the leading experts on the psychology of eyewitness identification). This applies both to the survivors and a single perpetrator who identified Demjanjuk. The latter identification is actually even more problematic, as Otto Horn was caught either lying about the initial identification procedure (which had been flawed), or misremembering it, which is just as problematic for his credibility as an identifying witness. (In fact, OSI tried to cover up the initial reports of Horn interrogation; this and other OSI misdeeds in the Demjanjuk case were characterized as "prosecutorial misconduct" and "fraud on the court" by the US Court of Appeals for the Sixth Circuit.)
So yes, Demjanjuk had been sentenced to death without any credible evidence whatsoever (even though the judges probably believed the identifications to be credible). But absence of evidence is not always evidence of absence, so how do we know that he wasn't Ivan the Terrible? It's simple: in postwar interrogations of dozens of Treblinka Wachmaenner another person is identified as Ivan the Terrible, Ivan Marchenko. Marchenko was an entirely "separate" man with a different bio from that of Demjanjuk and one of his daughters (Kateryna Kovalenko) was still alive in 1990s.
But there's still this silly ad hoc "alias hypothesis" going around. It is easily debunked by the fact of the existence of real Marchenko, who was Ivan the Terrible. Sometimes a useless bit of information is bandied about, namely, that Demjanjuk once listed his mother's maiden name as Marchenko, even though her maiden name was different. Marchenko is as common surname in Ukraine, as Smith is in the US. Actually, Demjanjuk claimed to have forgotten his mother's maiden surname, so it's no wonder he inserted such a commonplace surname instead.
But the main evidence actually lies in the fact that in official German documents Demjanjuk is always listed under his real surname. I.e., he was called "Demjanjuk" outloud by the Germans, both in the Trawniki training school and in the camps. Thus his fellow guards would surely know his real name. But not a single one stated that "Marchenko" was an alias of a "Demjanjuk". Also, since both the Germans and fellow guards knew his real name, there was no rationale whatsoever to use an alias.
Ivan Demjanyuk may have been "experienced and efficient" as a guard at Sobibor (his pal Danilchenko claimed so), and we will see in the coming weeks what the German prosecutors have in store regarding his role. But we do know that he never was a breast-slicing psycho from Treblinka.