"First, forget about paper records. DNA admixture analysis is completely independent of traditional research."
It is not independent on ancestry. They do name matching. And as you say, concerning the ethnicity "The labels are somewhat arbitrary". Ancestry is applying the DNA and providing names. Names are from paper documents. The DNA doesn't come with names.
Ancestry provides names from people who may or may not have good paper trails. And as I said, I believe they are assigning early people to certain groups. This will be reflected on down.
By the way, you really are one of the most helpful people here. I am not arguing with you. I am working through something that I think a lot of people are trying to understand.