I would say the presence of an exclamation mark, in a context where exclamation marks are rare, is strong evidence of use as an interjection. Unless we're arguing that some other mark was mistaken for an exclamation, generally I would say rare typography is "marked" (noteworthy) rather than being likely mistaken. I think the researcher's position is not likely to hold much sway going forward.

The exclamation mark is added in transliterations of the manuscript because it is believed to be an interjection. If you look at the manuscript, there is no such mark: https://en.wikipedia.org/wiki/Beowulf#/media/File:Beowulf_Co...

Why would you assume the original had an exclamation mark? Indeed the whole symbol was not invented until the 1300s.