Well, I do know that Duane Urch purchased several Buckeye pullets from me several years ago, so there's that. At the time he told me he was having trouble with (wait for it) fertility, and wanted to bring in some new blood to perk things up a bit.
I believe he also got a cock bird from Chris McCary at one point too, not sure when. Probably around the same time.
And I certainly think Mr. Urch would qualify as a breeder with an established line of Buckeyes.
As well, I have in the past sold Dutch Bantams to LeJean Marshall, another very longtime (more than 50 years) breeder of poultry. As with Buckeyes, most pure Dutch are very close in terms of their genetic connectedness, so it was certainly possible to make such "outcrosses" and see very good results. In fact, LeJean said one of the cock birds I sold him was the best he had used in years, and did great things in his pens. So there are two specific examples for you, right off the top of my head, involving my own birds going back to breeders with established lines.
Bear in mind, neither Mr. Marshall nor Mr. Urch frequent such online forums, so perhaps you are seeing results only from those who do, and your sampling is modified by that fact.
Just a thought.