They knew about the Kennewick man error fairly soon after discovery. Once the original forensic anthropologist found out they were ancient bones he knew and backtracked.
I agree that the age of the bones was resolved almost immediately.
The major issue was the origin of the bones - Europe or Siberia - which would have challenged the Siberian Land Bridge theory.
My recollection was that no DNA could be extracted because the bones were in wet sand for thousands of years.