Although I do not have access to the Article 10 mentioned in the question, I can still offer an answer about the word "unanimously" and the prefix:
In "unanimously", the prefix "un" refers to "one".
Latin prefixes are common in the English language. By definition, a prefix is a group of letters added to the beginning of a word in order to make a new word. The new word has a meaning that is related to the meaning of the original word.
For example, take the word "ordinary". When we add the Latin prefix "extra", which means "outside", to it, we form "extraordinary", which means outside the ordinary, incredible, amazing.
The prefix "un" can have two meanings, as mentioned in the question: "one" and "not". In the word "unanimously", the meaning is "one".
"Unanimous" means "<u>of one mind</u>". Thus, if a decision is made "unanimously", that means people decided the same thing, as if they had one mind.
