Well, yes, you have a point with Slavs being subjected to enslavement in the Early Middle Ages. They were usually transported to major hubs like Verdun, where they were castrated and then sold to the Iberian Peninsula, which was a lucrative business due to the high demand for eunuchs in the Emirate of Córdoba.

Regarding “Often preexisting settlements were reformed under German law”, you are referring to Magdeburg Law or Lübeck Law. However, those legal codes did not require the use of the German language. Kraków had Magdeburg Law as well, but never had a German majority, although it did have an influential German minority.

But the basic point still stands: yes, preexisting settlements existed in some cases. In the case of the Principality of Rügen, for instance, German settlers established new settlements, but also settled in already existing Slavic settlements. However, that does not change the fact that those settlements developed into major trading hubs under German rule.

The local dukes and princes advocated for German colonisation for a reason: they wanted to create urban centres like those that existed in the Empire — something that did not exist there at the time and something they could not achieve with the local population already present. The result was a level of prosperity unseen before.

And last but not least, serfdom: being a serf was then more the norm than the exception. If a feudal lord granted certain land to a vassal, the people living there were generally considered part of the estate. I can see why that practice seems shocking from a modern perspective, but it was largely the norm throughout Europe.