Once you trace back centuries, in my case early 17th century, you run into issues of everyone having the same name, scarce scrapes of documentation, and other issues that can produce more than one plausible ancestor line. This ancestor was in successive junior lines so it's a lot of implication -- showing a pedigree chart with "Henry" born to "William" in 17th century England -- that isnt very helpful without piecing together a lot of scattered context from multiple sources and without being able to see what documents/evidence those researchers had.

So, theory is a common term used. Established theory because the family is included in 3-4 books written from the late 19th and 20th century covering large swaths of the related families and areas/interactions/movements etc.

Once you start getting back that far in time the people who trace genealogy for money and writing books, tend to be more through and understand the quirkier nuances that can tease apart families that your typical internet sleuthing researcher (me for example) can miss/conflate.