Hm where did you find that info? "It is" +adjective -types of sentences can use either the partitive or the nominative depending on the subject. If the subject is a countable thing then we use the nominative and if not, we use the partitive. Because of the "Yum!" we can assume that the sentence is about food which is usually uncountable.
Here is a good source on the use of partitive and also the formation: https://uusikielemme.fi/finnish-grammar/grammatical-cases/the-partitive-case-partitiivi/