[I started with a brief mention of name mentioned in the question.]
If you have another look at the guidelines you will see they say:
The reason for saying this is that UK genealogists regularly get queries from the U.S.A. which say - without quoting sources - that they know their UK ancestor came from a particular place at a particular date. In many cases it turns out they have never even looked at an original handwritten document of the period (or a suitable digital image). The information they have has merely been copied from a family tree produced (without quoting sources) by someone else. In many cases it would seem that the tree had been copied again and again. The original printed tree was probably produced 100 years or more ago, when modern computer indexes were not available, and research was very much harder (and expensive) to do properly. What was originally a best guess or a downright error is now considered to be TRUE because the original guess/error has been repeated on hundreds of web site by people who knew so little about proper genealogy that they didn't know the importance of checking and quoting sources.
I don't know if this applies to you but in your case there are regular queries about William and I know that they have been looked at in detail by an experienced family historian who has drawn a blank. You might well be advised to accept that you are unlikely to be able to trace the paternal line any further back. with any degree of confidence.
Be realistic. All genealogy investigations are bound to end with a complete block at some stage - some people who are adopted may not even know the identity of their own parents, while a vary large number of people can link into the nobility (where there are many published trees - not all accurate), and hence into the well documented royalty of Europe - so that there must be many millions that have a family tree in which at least one line could be traced back to Charlemagne.