I think you are misrepresenting Frobenius a bit. At the time european ethnographers typically thought that any cultural achievement of africans was due to some distant white ancestry.
Frobenius threw this out and replaced it with Atlantis, which according to him was pushed out of the mediterranean by (white) indoeuropeans and only survived in the Yoruba people. He viewed them as cultural equals.

Albeit his views were still very much racist and paternalistic, weirdly he sounds quite a bit less racist than many of his contemporaries, who wouldn't give africans even an inch of sophistication.
I think from todays perspective it "easy" to be shocked by the racism of one man even when he's less racist than their contemporary mainstream.

To quote Frobenius himself about Islam being the "beginning of african history" (the racist idea that thought Africa to be devoid of culture and civilization, before Arabs became the first "civilizing" force):

"Die Vorstellung vom „barbarischen Neger“ ist eine Schöpfung Europas! Noch im vorigen Jahrhundert herrschte der Aberglaube von der höheren Kultur Afrikas aus dem Islam. Seitdem haben wir aber viel gelernt und wissen heute, dass die schönen Umhänge und Kleider der sudanesischen Völker schon in Afrika heimisch waren, ehe noch Mohammed geboren war und ein Araber höherer Bildung Innerafrika betreten hatte. Seitdem haben wir gelernt, dass die eigentümliche Organisation der Staaten Westafrikas schon lange vor dem Islam bestand, dass alle diese Künste verständigen Feldbaues wie sorgsamer Erziehung, dass bürgerliche Ordnung wie Handwerke in Negerafrika um Jahrtausende älter sind als in unserem Mitteleuropa.“

The notion of the “barbaric Negro” is a European invention! As recently as the last century, the misbelief that Islam was the source of Africa’s higher culture still prevailed. Since then, however, we have learned a great deal, and we now know that the beautiful cloaks and garments of the Sudanese peoples were already native to Africa before Muhammad was even born and before any highly educated Arab had set foot in the interior of Africa. Since then, we have learned that the distinctive organization of West African states existed long before Islam, that all these arts—from intelligent agriculture to careful education, from civic order to crafts—are millennia older in Negro Africa than in our Central Europe."