Trocken wines are not devoid of residual sugar, but have, at most, a few grams per liter, which can be perceptible but is not overtly sweet.
The maximum amount of sugar allowed for a trocken designation depends on the level of acid in the wine.
[1] Most high-quality German white wines have a high enough acidity to be allowed up to 9 grams per liter of sugar under the trocken level.
[1] This parallels the term sec ("dry") in French, which indicates a sparkling wine of the same sugar level as trocken.
While it appears in the term trockenbeerenauslese, trocken in that case refers to the dried grapes, not the dryness of the resulting wine.