Comment by gjm11

Same purpose, different grammatical structure.

Heaney's famous translation begins "So. The Spear-Danes ..." with that "So" being an interjection, a thing that could in principle stand on its own. (You might say "So." and wait for everyone to settle down and start listening.) Even more so with things like "Yo!" or "What ho!" or "Bro!" or "Lo!". (Curious how all the options seem to end in -o.)

This is more like "So, the Spear-Danes ..." where the initial "So" has roughly the same purpose of rhetorical throat-clearing and attention-getting, but now it's part of the sentence, as if it had been "As it turns out, the Spear-Danes ..." or "You might have heard that the Spear-Danes ...".

I think the theory described in OP makes the function of "hwaet" a little different, though; not so much throat-clearing and attracting attention, as marking the sentence as exclamatory. A little like the "¡" that _begins_ an exclamation in Spanish.

Of course a word can have more than one purpose, and it could be e.g. that "hwaet" marks a sentence as exclamatory and was chosen here because it functions as a way of drawing attention.