How can they say with any real conviction that Hengest was not real? Of course, it is understandable to suspect that he and Horsa were not real persons, but there again we are 1500 years away from when they were supposed to exist.Without wishing to reignite old debates, this is not strictly true. The overwhelming prevailing opinion within academia is that Hengest was not a real person. He is not considered a suspect at all, let alone the prime one. The greater use of multidisciplinary approaches and a move away from both narrative history and the study of early medieval history in an entirely insular context has led to a radical reappraisal of the post-Roman centuries.
Outside academia, I accept that people are far more ready to believe that Hengest was real and that the deeds attributed to him in our written sources are true. However, for the most part, that belief is rooted in the academic consensus of previous generations, which reached its high water mark in the 1970s and which has been steadily in retreat ever since. As more and more earlier scholarship falls out of copyright and/or is made available for free online, the easier it becomes to get a snapshot of how things were seen in the post-War years, rather than how they are seen now. Getting up to speed with current thinking means getting hold of expensive academic textbooks or journals which are often not freely available (or available for free) outside academic libraries.
No-one is saying that everyone has to accept the new consensus, but it is important to flag up that it exists.