TLDR: Starbucks are using the term "Sugar-Free" when they mean “no added sugar” although there's sugar naturally in the ingredients.
I'm not sure how anyone can argue that isn't clearly breaking FDA guidance of containing less than 0.5g of sugar:
https://www.fda.gov/regulatory-information/search-fda-guidan...
This is basically standard practice worldwide for most "sugar free" products. Most common sources are dates, apple juice concentrate, honey in my experience.
Products actually using erythritol, mannitol, or xylitol instead are a rarity.
It seems to go further. They must be intentionally adding ingredients that have a high sugar content, else they would never get to the 21g of sugar in a 21oz drink, as mentioned by the article.