The book cited provides a host of new perspectives on Abraham Lincoln. His talk to the assembled Indian chiefs from the plains is just one item among many others. Personally I'm comfortable with most of the author's conclusions because he presents himself as a Lincoln supporter. The Plains Indians heard an adaptation of a well-worn speech on American innovation Lincoln made several times before becoming President. Don't know if he used the term "Mexican greasers" in each version, or if that was added for his Native American audience.
This is just one of several Lincoln speeches were are unlikely to hear schoolchildren recite.