Thats how the study justifies it:
Our newly reported data also revealed sporadic long-distance mobility. Three men who likely lived in the 2nd or 3rd centuries CE fell outside European and Near Eastern variability (
Figure S1), close to present-day and ancient Africans (
Figure S2A). Proximal
qpAdm modeling confirmed these observations (
Figure 2A;
Data S2, Table 6) with 33% and 100% North African ancestry for individuals
I26775 (
Iader) and
I32304 (
Viminacium Pećine), respectively, while
I15499 (
Viminacium Pirivoj) could be modeled using only ancient East African populations, supporting an East African ancestral origin and agreeing with his uniparental markers mtDNA L2a1j and Y-chromosome E1b-V32, both common in East Africa today
28,
31. The individual of East African ancestry was buried with an oil lamp depicting Jupiter-related eagle iconography (
Figure 2C;
Data S1,
section 1), not a common finding in
Viminacium graves
32. Isotopic analysis of tooth roots showed that he was also an outlier with respect to dietary habits during childhood (
Figure 2B), with elevated δ15N and δ13C values indicating the likely consumption of marine protein sources
33, unlike individuals from Pirivoj and other necropolises whose values (
Figure 2B) were similar to the Roman-Period population from
Sirmium34 and consistent with a largely C3-based diet with a significant portion of animal protein consumption
33. Thus, he likely spent his early years elsewhere, possibly in East Africa, the land of his ancestors; while we will never know his whole life story, whether as soldier, slave, merchant, or migrant, it encompassed a long journey that ended with his death in adolescence on the northern frontiers of the Roman Empire.
I think also that these samples were probably some mercenaries,and an african migration wave didnt really happen
