I'm a pretty talented amateur genealogist. A friend asked me to help her find her birth parents on Ancestry, through her DNA test. We found her mother quickly through first cousins. The father was tougher, for a number of reasons. Working through others connected to her DNA but not her mother's, we found her father's family. I worked on 4th cousin matches for weeks, to no avail. Another friend using a different method finally located the father's family, and then I found he had been missing from my tree. I added him, researched him some more, and voila! there he was.
My point is this, and these hunches are almost as good as fact 1. Once they had the cousin match, they used all the genealogy websites they could to build their own family tree, with as much information as possible. 2. They hired tree experts. A novice could have never traced the 4th cousin match. You need a ton more information, research skills, and imagination-- including info on living people-- which is not just on genealogy websites. FWIW