It's neither ambiguous nor confusing to use the word union in CS. The only person who's making it so is you, by introducing semi-unrelated concepts from set theory that happen to have the same name as the established CS concept.
Why stop there? Maybe the author meant the Union, as in the United States? Itself quite ambiguous - does he mean the United States of Mexico, or the United States of the Ionian Islands? Is C# getting Corfu? Corfu dot net? :P