The sentence that requires translation is "Το παγωτό είναι γλυκό." It includes the direct article. Therefore, it's referencing a specific ice cream. The answer should also include the direct article. Because it's a word-select question/puzzle translation without the article as an option to select, it's incorrect.
Technically, dessert is "επιδόρπιο". Sweets can be eaten as dessert, but you wouldn't call a healthier dessert (for example fruits with yogurt) "γλυκό". So, they shouldn't be used interchangeably, even though "επιδόρπιο" isn't used as much, and - let's face it - we most probably are going to choose a sweet for dessert! :)