That would only be slightly less anachronistic than extracts from "Camelot". We simply don't know what people sang about at that time, or what it might have sounded like, and we will never know. Specialists (which we are not) can make more-or-less educated guesses, but that's all. Of course, the average novel about the period will be full of anachronism (I've read some truly ludicrous ones in my time) but there's no need to make things worse than they need be. Sarah needs to be talking to musicologists, not folk music enthusiasts like us.