You're talking absolutes, which is neither possible, not the point.
If taking any random sample of 3 people in e.g. Roman Britain, you had e.g. in 1/1000 chances of having a black person among them, then showing 3 persons and having 1 be black is not a representative picture. It's that simple.
I'd argue a large reason we're having this discussion at all is exactly because the idea of black people in Britain at that time is surprising to a lot of people, exactly because a lot of people have an image of the Romans as being mostly white Italians to the point where seeing a black character at all is a surprise, rather than out of concern that they're not accurately representing the precise proportions.
This shouldn't even be an argument, since history over-represents minorities all the time. It's not a mystery as to why. Trade and war have a couple of things in common. They feature prominently in histories because they are a catalyst for change, and they are practiced as often as not by "minorities" - ethnic, religious, racial, etc.
Tell a history well (or poorly) or try to make some kind of historical point and it is likely you are going to be talking about minorities. Over-representing them, to be sure. Taleb knows all this, and his recommendations regarding written histories have been pretty good. Regarding this thing, he was just being a combative douche. (or he has some other goofy racial agenda, as some have speculated... who knows)
“Accuracy” is not possible if your depict a smaller group then the whole, as any story (regardless of media; visual, text, or otherwise) that focusses on a small number of focal characters to illustrate something about broad society and time period will not accurately represent the composition of the broader society.
You either exclude lots that was present by focussing exclusively on the most common or overrepresent minorities that were present in the broad society by having them present at all in the small focal group, and in practical terms you end up doing some of each. But you can't losslessly compress history; you just have to choose what's appropriate for your purpose.