People are saying that the interjection interpretation is influenced by its use as interjection in Shakespeare’s time. By that time what/hwæt was being used differently than the way it was when Beowulf was authored hundreds of years before.
"Det er sant." That is true. "Det er vel sant." That's true, I suppose (resigned). "Det er nok sant." That's true, I suppose (serious). "Det er da sant." That's true, come on.
German also has them, though I'm not confident enough to explain the fine difference between "Das ist wahr" and "Das ist noch war".
English maybe has some remnants of them, but they're rare and probably a bit more archaic. You can say "That is yet true", and the "yet" there doesn't necessarily imply that you think it might not be true in the future. You probably understand what I mean if I say "That is but true" but it sounds very archaic. Usually you have to invoke a full adverb to signify mode in English ("That's actually true")