The term "Moor" does not translate precisely to any modern ethnic term, and is perfectly compatible with dark skin. In fact, the article notes that "The unit is likely to have been composed of Berbers from North Africa, but may also have had darker-skinned soldiers from Nubia."
http://www.telegraph.co.uk/news/uknews/1464209/Borders-folk-...
>The lack of evidence (genetic or historical) puts an upper bound on the proportion of subsaharan Africans in Roman Britain
It does no such thing. Can you point to any reputable scholar who is prepared to state such an "upper bound"?