Why does Beowulf refer to Freawaru Ingeld and the Heathobards as he boasts to Hygelac about his great deeds in the land of the Geats?

The references to the wedding of Freawaru and Ingeld are clearly ironic. Beowulf is talking of the dangers of a marriage made to solve a peace. Presumably the audience of Beowulf would have been aware of other stories that we are not aware of and can only glimpse, wherein the events that Beowulf postulates actually happen. From the poem Widsith we know that Hrothgar and Hrothulf (his nephew) were renowned for having defeated Ingeld at Heorot.

Beowulf is therefore being a "prophet" in that he is referring to events that from his POV have not yet happened, but the audience will appreciate the reference to another (lost) poem.