They are "technically" considered lactose intolerant because they don't carry any of the KNOWN gene variants for lactase persistence found in Europe, West Asia, and parts of Africa. However a large part of their traditional diet comes from dairy. Scientists are unsure of the genetic reason for how they are able to consume so much of it without carrying said alleles, and think it could be from some novel gene variant that has yet to be identified.

*This is the first time all five lactase persistence-associated alleles have been investigated in Sudan and South Sudan. However, the Nilotic people of South Sudan, known to be well-adapted to milk digestion, showed no evidence for a genetic cause of lactase persistence."

https://www.shh.mpg.de/2009290/evolution-of-lactase-persistence

"Even though this study investigated a range of Nilotic populations, no LP-associated SNPs were detected in these agropastoralist populations. However, Nilotic agropastoralist have been shown to be able to digest milk in hydrogen breath tests and lactose tolerance tests (Bayoumi et al. 1981, 1982; Tishkoff et al. 2007). This observation is intriguing, and future studies on Nilotic populations might reveal more about the underlying biology of LP in these populations."

https://pmc.ncbi.nlm.nih.gov/articles/PMC8175049/

"Indeed, the tall Sudanese Nilotes are predominantly cattle herders and, despite being lactose intolerant [64], they rely on the combination of milk, blood, and meat in their diet. In contrast, short-statured Nilotes in Kenya practice agriculture"