What are the confusing words? You gave a quote that sums up the long-accepted understanding of who's exempt from the citizenship clause:

"This will not, of course, include persons born in the United States who are foreigners, aliens, who belong to the families of embassadors or foreign ministers accredited to the government of the United States, but will include every other class of persons."

They don't mention Indian tribes in that quote, but they mention it elsewhere. Seems completely clear to me.