Nothing new here. Basically the genetics, matches the historical accounts. We know (by historical accounts) that the slavic immigration started in the end of the 6th century, after the death of Justinian the Great, where the successors lost territories and few battles and could properly defend the frontier. (avar wars).

Also, after the Justinian plague (541–549 AD) a lot of the local population was decimated, hence the new slavic arrivals found it easier/space to move to the country side (initially as a trickle of immigration in the late 6th century, then eventually as settlers/invaders later in the early 7th century).