Dudo appears to have been a fairly confused chap. He also seemed to confuse "Danish" and "Dacian".
However, as has been noted in the comments, it a reasonably common theme in national mythologies to suppose some honourable (and no longer extant) origin for a people. Troy has been popular, with the most well known example being the Romans through the Aeneid, and of course Snorri's claim regarding the Aesir.
By some interpretations, the Danaoi are not, however, equivalent to the Hellenes. The Hellenes were the people of classical Greece as we generally understand it, while the Danaoi were the previous occupants of the region who were culturally supplanted by the incoming Hellenes. There is even some evidence to suggest that they were the people of the Mycenaean hegemony that predated that of the Greeks. One might then construct an idea, based on that folk etymological leap of Dudo, that some of the Mycenaean peoples made their long way to northern Europe, possibly in association with the decline in prominence of their own culture.
There is, to my knowledge, no evidence of this.
Of course, a little after Dudo's time there was a cultural link between Scandinavia and the Greeks of that time. Norse longboats were apt for river navigation, and their crews undertook expeditions deep into central and eastern Europe by that route - and by portage made the crossing to navigable rivers by which they could continue downstream to Greece and even the Black Sea. This may have been important in the development of the Byzantine Varangian Guard, a force dedicated to the Emperor that was first made up of troops lent by the Kievan Rus but later largely composed of Scandinavians. They would go to seek their fortune, and many would come home. Thus there were trade links via longboat, and cultural links via this traffic in young men.
Ultimately, you will never get an authoritative answer, at least until we develop some method of looking clearly back in time, of what Dudo really thought - much less of what might actually have happened. This was perhaps not by nature of a foundational myth for the Danish people themselves, as Dudo was simply commissioned to make a history; he may be reporting oral traditions, or may be making up large amounts from whole cloth. I can't answer what explanation Dudo might make, because I don't have a copy of his work (and the properly translated and annotated copies cost a pretty penny), so this is not, if we are to be strict, an answer to your question. However, I thought there was enough relevant information and exposition that I might offer that it was worth taking the space here to offer it as an answer.
