You used the UK as an example, however, Romans, Saxons, Vikings and Normans were ethnic Europeans and these genetic groups didn't split from indigenous people in the British isles tens of thousands of years ago but more recently. The projection (not fact) above mentions that: "in a few decades the genetic makeup of most Europeans will be more similar to that of NON-European ethnic groups than to that of people who have lived in Europe for TENS OF THOUSANDS of years".
In any case if you were any of those groups, you would have opposed an invasion from a later group. Picts fought Roman "immigration". One might interpret this as Picts wanting their genetic makeup to be more representative in the overall population of the British isles.
I don't see how this justification might convince someone who is "an ethnic European and predominantly carries genes that split from the ethnic groups of immigrants tens of thousands of years ago" that the projection doesn't matter if they don't already believe so.
I wasn't aware that these extremists aren't concerned about genes. I haven't read the manifesto nor am I planning to. However, people can still have a go at debunking the genetic argument above if they wish.