Can that be argued? It's not that the "contact" and influence didn't exist, it's just that these specific people wouldn't use that language at that moment of time. The Romans were already on English ground long before Beowulf was written so using Latin words there is still much more acceptable to me -- it doesn't imply some access or historical impossibility that didn't exist.
The same can't be said about sashimi. Again, like in the example of the movies (in other messages), I'm simply not expecting absolute purity, I just find Seiko watches too distracting there.
It is not a Latin word, it is a word from Old French which is a Romance language evolved from Vulgar Latin. A lot of words from Old French entered English after the Norman conquest. This is the major difference between Old English (the language of Beowulf) and Middle English.
You may not care about any of that, but my point is a translation into modern English is by definition anachronistic. Avoiding 20th century loanwords like sashimi will not make it authentic, at best it will make the text feel slightly old fashioned, but it would still be wholly anachronistic.
For the reference, the Latin word is monstrum, and like I've said, there were surely people using that word while living on the territory of today's England long before Beowulf was written.
I'm quite sure no Japanese were living there either before or at the time Beowulf was written. That's the difference that I consider jarring, not the fact that the newer language is the language of the translation. Even in English, not all words have the same "weight" -- some simply "sound" new and have certain "modern" connotations and some don't. Some imply origin from one place, some from another, etc. It's simply more complex than "the whole English is newer, who cares" approach.
My favorite example from 1490 is:
"In this book, Caxton tells the story of some merchants from the North of England trying to buy eggs from a woman in the South of England. The northerner uses the word egges, derived from Old Norse, but the Southern woman, who uses the word eyren from the Old English, does not understand. A humorous misunderstanding ensues."