And why would Poles be taking on a nickname Polak given to them by foreigners on Polish territory?
The existence of the surname Polak in Poland doesn't imply it is a redundant ethnic self-identification or was foreign directed.
Serfdom did exist in the Poland up until the 18th century. Although there are accounts of land-owning magnates and szlachta regarding themselves as ethnically distinct from the peasantry who labored for them these nobles were largely Polish themselves along with Lithuanians and some of Ruthenian origins (the latter two groups which became Polonized themselves).
Despite the existence of a large social strata back then the nobles were not foreigners. Given the common usage of Polish as well there is no basis for anyone in the upper echelons of Polish society back then referring to any of their labors as a Polak and Poles in turn adopting this as a surname given to them.
There were also periods when Poland was under foreign occupation such as by Austro-Hungary, Russia, Germany and then the Soviets. But the seizure of Polish territories by these alien hegemonies occurred when surnames were already well established in Poland. These aforementioned occupying powers were not Polish speakers themselves and there is no evidence that they ever held such social and economic influence that they were in a position to refer to any natives Poles as a Polak and then Poles obsequiously adopting the term as a surname.
The surname Polak likely stretches further back in antiquity. Since "pole" is the Polish word for "field" the adoption of Polak could have been completely innocuous such as to refer to someone who worked in, owned, or resided in or near a field. For instance, it could have been given or adopted to distinguish two people named Jan one who lived within a village and one who lived outside it in the surrounding fields.
In other European countries people have similarly simple topographic surnames such as Field (in English), Feld (in German), Champ (in French), Campo (in Italian), etc.
