Mathematics makes this a crushing revelation.
It does not mean there were 20% conservatives left on the list. It means there were 20% conservatives left on the list to be vetted. We don't know, and never will know, who failed vetting. It would be horribly rude to release a list of those who failed vetting. Every nanny tax violation. Ever pot smoking misdemeanor. That's a lot of lives ruined just to explain to an intense base why one of their rockstars didn't make it through.
What we see is, indeed, Miers was the most qualified available candidate. With confirmability as a major qualification, it's hard to argue with "most qualified available candidate".