I'm skeptical too, but the "selection" would occur as people died before being able to reproduce, whether they chose who they reproduced with or not. Of course lots of European genes were introduced by slaveholders and their overseers.
Overseers, yes. Widespread antipathy still toward Scotch-Irish, who comprised the majority of overseers. Owners? Not so much as pop culture and politically motivated historical revisionism would lead one to believe. It happened but was not all that common.
Scan back upthread for the link provided by AdmSmith, which provides the most common means of introduction.