why do we use "c'aussi" to say "it was"?

I don’t have access to the exercise, but as it is apparently an historical piece, an  explanation may be in the usual use of present tense for narrative aspects in French history writing, whereas English usually writes historical pieces in past tense. 

As this is a B2 exercise, this technical knowledge is possibly expected, and the original text and translation are each appropriate to their own language.  https://www.lawlessfrench.com/grammar/historical-tenses/

I haven't done the exercise either, but according to the instructions it uses the passé composé, so I don't think this is the answer. Both the examples that Carl mentions use "c'est", so I'd assume this is what's relevant.

Alan, the instructions simply say to use passé composé, not passé simple (when past tense is indicated). French historical writing still uses past tense for appropriate specific events, but the narrative is mostly in present tense. The fact the recommendation is there suggests to me that otherwise “traditional” French historical writing is to be expected in the piece. 

Well, someone on the Kwiziq staff could confirm this either way, but I have a feeling that they sometimes accept our answers (and mark them "correct") without checking.

Wouldn't it be a bit surprising to mention the passé composé, but not the présent historique? And isn't it a coincidence that both examples use "c'est"? I've pointed out, previously, that "c'est" is a special case, and is often used in the present when you might expect a past tense.