It would be very helpful to find where Douglass uses the term "Anglo-Saxon" in his Composite Nation speech. I could not find it in the Library of Congress' microfilm typescript. I did find Douglass' references to "Indian and Celt, negro and Saxon, Latin and Teuton, Mongolian and Caucasian, Jew and gentile". (page 33). And, of course, he refers to himself as an "Ethiopian".
The term "Anglo-Saxon" only came into popular use in Britain at the end of the Victorian era as a moral justification for their Empire and a useful catchall for the English-speakers of those islands that helped paper over the very real differences between the English, Scots and Irish. But, no American politician would have gotten very far in 1867 as an advocate of "the idea of that America was an Anglo-Saxon country". That would have been electoral suicide, as Douglass himself would have been the first to point out; how could you unite "white" voters as a racial bloc by choosing a term that insulted the second and third largest groups, by heritage - the Irish and the Germans?
If we are going to stick to the year 1900 as the cut-off point for historical data, it seems to me we need to apply the restriction to attempts like this one to drag back to 1867 modern ideas that were not in the minds of anyone back then. We could follow current medieval studies departments in abandoning the use of this manufactured term. As myth the notion had a brief flurry of success at the same time people were building statues to Cecil Rhodes. As a term in common use, even Henry J. Ford, the illustrator of Andrew Lang's Fairy Books, thought it was a complete fiction.
https://en.wikipedia.org/wiki/Henry_Justice_Ford