Hi, hopefully someone will be able to address your question directly, but meanwhile you may be interested in these, which touch on the subject

• /u/rimeroyal in Why does Beowulf, an Anglo Saxon epic, feature Geats and Danes but no Angles or Saxons?

• /u/Starkodder mentions the Sam Newton theory re the East Anglians in [Medieval Norse/Danish/Anglo-Saxon history] What do we know about the history of the people group the Beowulf poem calls "Scyldings" after the "events" of Beowulf?

I'm far from an expert on the subject, but I'd assume it's because Beowulf takes place in the first half of the 6th century and was written down in the 7th or early 8th, before the start of what we consider the Viking age (which started in earnest in the late 8th/early 9th century).