Tbh a lot of chicken genotypes are only theorized, not fully understood.
Only a few "solid buff" birds were ever genotyped, and they never even genotyped the "token buff" breeds, Orpingtons and Cochins.
https://www.backyardchickens.com/threads/buff-genetics.73980/post-795855
Certainly, it's possible there is more than one means to an end and not all buff birds have all the genes we think are in buff. And maybe some have more.
And it could be the same with red, another complex color.
And solid black can be created in so many ways it's also difficult to grasp.
For example, this bird (and photo) by Blake Bell
View attachment 4083894
His parents are a buff Leghorn and Black Minorca. This shouldn't happen. Perhaps black Minorca are Birchen and not extended black based?
Honestly, considering Nicalandia said people think they
might be Birchen based (or at least some might me I believe that is the answer to our mystery here
https://www.backyardchickens.com/threads/genetics-question-pure-breed-x-hybrid.745611/post-10492936