TL;DR Your mileage may vary.
Only Human offers a short introduction of different ethnic groups and languages of Rûl, a human variant, and new tables to create a slightly more detailed background for your human character.
Pages 2-5 discuss human origins, languages, ethnic groups, and their attitudes toward the fantasy races. If you play SotDL in the core setting, you might find value in this.
Rest of the pages (5-14) are dedicated to a variant (barely) human ancestry, where you just roll 8+d3 for your stats instead of taking 10, and rather detailed tables for your character's background. You can roll for your family, appearance, life experience so far (childhood, setbacks, windfalls, strange events), and there's a streamlined bunch of tables for human professions. Several of them give you starting equipment. The tables aren't dissimilar to the ones you already have in the core book.
Overall the product isn't bad but it's too expensive for what it is since you get more-or-less the same stuff you already have in the core book.