No need to get "barking mad", just point out the details, refer to sources if you have them, and make or request changes. I am not sure why this line is mastered. It was likely done to prevent merge errors or vandalism, since there are no sources listed in Geni.
Also, it's obvious this is not a result of one person writing "fantasy", as "William" himself has 18 managers. It may very well be a result of 18 people copying misinformation, but there is no reason for you to assume someone created misinformation on purpose.
I will search for sources and do some work correcting these lines very soon. Thanks for bringing this to our attention. If you could point me to any sources that you know about, that would help.
Also, mentions of the UK when it could not have existed are a result of Geni confining location fields to names available in Google Maps. It's difficult to get Geni to accept a place name Google Maps doesn't recognize, and even if one spends the time doing it, the next person's merge or data resolution may overwrite that more correct name with modern names. This issue has nothing to do with this profile or its managers.