What's confusing you is that this is not a past continuous construction.
In English, the gerund can be used to form a past continuous with "was":
He was walking down the street, minding his own business, when the grapefruit hit him.
However, it can also be used to create a verbal noun:
His hobby is sitting.
If we are talking about the past, we might form the past tense with "was":
John was a lazy bastard; his hobby was sitting.
But that doesn't change the fact that "sitting" is acting as a verbal noun. It is not a past continuous.
In the same way, your sentence is using "seeing" as part of a verbal noun phrase. For example: if you were to say:
What inspired us was heroin.
"Heroin" is a noun, the object of the sentence, and the verb in the sentence is "was."
Similary, when you say:
What inspired us was listening.
"Listening" is a verbal noun formed with a gerund, and the verb is still "was," not "was listening."
Tack on a prepositional phrase:
What inspired us was listening to our flatmates
"Listening to our flatmates" is a verbal noun phrase. There's no implication of continuous action here; the verb is the simple past "was."