So the reality is that we know there has been people here that are poorly reflected in the genetic record, and that makes it rather pointless to point to the genetic record to demonstrate the family in question is implausible.
In any case, what troubles me much more than whether or not the depiction is plausible is just how incredibly unprofessional and rude Talebs reaction was. If he wants honesty and a focus on the science, he should grow up and act professionally.
There is no reason to think that a few interracial marriages in Roman times would have left much of a trace in the modern genetic record. Surely no-one would get so up in arms about a cartoon that showed a Saxon married to a Norman, but we have no modern "genetic record" of this either. We know with a reasonable degree of certainty that interracial marriages existed in Roman Britain, often involving soldiers.
Talk about the "genetic record" might seem sciency sounding, but its really the historical evidence that's much more informative in this instance.
Beard seems to say, "it was possible that it could have happened, here are a few Ethiopians for instance that we know were in Roman Britain, for example".
Taleb seems to say "typical means what is likely to have happened 50% or more of the time; since there was no chance that Roman Britain had 50% of marriages with children that involved Ethiopians/Berbers/etc. thus Beard is very wrong"
The reaction, and Beard's answer, was specifically about the family shown and not the narrow question of whether or not an Ethiopian conscript would have ever set foot in Roman Britain.