I don't see anything in the article proving that Confederate Memorial Day commemorated Union, as well as Confederate, soldiers. Do you? That's the lie in the article:
Memorial Day in America was first held as “Decorations Day” in the South in honor of both the soldiers of Confederate gray and Union blue.
I did find out elsewhere that an early memorial celebration was held by Charleston African-Americans in commemoration of the Union dead, but I've yet to see anywhere that Confederate Memorial Day involved tributes to the Union, as well as the Confederate dead or that celebrations of a Confederate Decoration Day had anything to do with the graves of the Union dead.
I only skimmed the article, but it seemed to start out referring to Decoration Day / Memorial Day (when the graves of both blue and gray were tended to), then shifted to discussing the later Confederate-only memorial date. I concede that the beginning of the piece doesn't mesh well with the article title.