The americas are really big

The size doesn't matter.

  Grouping every person "pre columbus" into a single bucket is wrong

Wrong for what? To make a distinction between to generalized groups can be a very useful abstraction in language, debate and science. "pre columbus peoples of the Americas" is already way more defined then "ethnically american" which was the main point I am making, "ethnic" is a vague term.

Its as good as you are going to find in grouping people by ancestry.