You are right it is a newer adaptation, but one that was clearly advantageous to our species. There are not many evolutionary changes that occurred independently in 3-5 different populations.

https://en.wikipedia.org/wiki/Lactase_persistence

https://pmc.ncbi.nlm.nih.gov/articles/PMC3048992/#%3A~%3Atext=Lactase+is+the+enzyme+responsible%2Ctrait+known+as+lactase+persistence.