The making of the Slavs - which "slavs"?

I'm not sure if this belongs to the "Medieval Europe" section or not, but since it is mainly about a interpretation of a book I post it here first.

Anyways, I've been looking forward to read Curta's The making of the Slavs, and I will have the pleasure soon since I'm waiting for my local library to have it in stock. After reading up on some of the reviews on both Amazon.com and on other sites a question have come into my mind all the time.

According to one review, on page 3 Curta writes:

Instead of a great flood of Slavs coming out of the Pripet marshes, I envisage a form of group identity which could arguably be called ethnicity and emerged in response to Justinian's implementation of a building project on the Danube frontier and in the Balkans. The Slavs, in other words, did not come from the north, but became Slavs only in contact with the Roman frontier.

Now I do not intend for someone to write a whole essay on the subject (since I'll have the book in about two weeks), but my question is: If Curta does not speak of a slavic migrations to the Balkans, who were the people that would in the future become the south slavs? Were they paleo-balkanic inhabitants who over time adopted slavic speech? What did it take to make so much of the Balkans into "slavic area" if there were no slavic migrations to the area?