In Aristotelian logic, dictum de omni et nullo (Latin: "the maxim of all and none") is the principle that whatever is affirmed or denied of a whole kind K may be affirmed or denied (respectively) of any subkind of K. This principle is fundamental to syllogistic logic in the sense that all valid syllogistic argument forms are reducible to applications of the two constituent principles dictum de omni and dictum de nullo.
In Aristotelean syllogistic, these two principles correspond respectively to the two argument forms, Barbara and Celarent.
These principles correspond roughly to a valid argument form known as universal hypothetical syllogism in first-order predicate logic.
This by itself accounts for why it is incorrect to identify Dictum de omni as universal instantiation.
The latter mistake is more easily explained by the fact that universal instantiation is a single-premise form of deduction that is not even a syllogism.