Alan is correct. The initial answer here that was indicated correct, was wide of the mark, to say the least. Apologies for the brain implosion, misdirection and confusion !

Deleted. 

Am I missing something here? Isn't "était ajoutée" the imparfait, passive mode? The verb ajouter is conjugated with avoir.

Andy,

The text is using present tense in this section - common in French ‘ historical’ writing and narration, whereas English regularly uses past tense. 

Hence ‘ est ajoutée ‘ ( in 1972 ), passive voice, present tense, taking up Alan’s comment. 

There are other examples in the lessons here, but have a look at the link about Coluche, for further examples of present tense describing notable events happening at a particular time in his life  eg in 1985 - ‘ il crée …. ‘, ‘ (il) demande …’ 

 https://www.lawlessfrench.com/grammar/historical-tenses/

 Forming La Voix Passive with simple tenses in French (French Passive Voice) 

Forming La Voix Passive with simple tenses in French (French Passive Voice)