That’s a fair argument. On the other side, Porter (O. Henry) was plainly influenced by French short-story master Guy de Maupassant (1850–1893). Porter may well have borrowed some of his famous twist endings from Guy.
Having just read a lot of Porter (see Marketplace for my edition of Sixes and Sevens) I can report that he went further with the “characters speaking regional dialects” than fits modern tastes. Like Twain, Porter traveled widely in the USA and employed what he experienced to create Local Color in his stories. And as @Melbourne points out, 1800’s humor that was distinctly American went along with all this.