It's the same as in English, where a couple is a singular grammatically, but because it consists of two people you may sometimes treat it as a plural:

The couple next door are fighting again.
The couple next door is getting married next week.

You could say that using the singular puts a focus on the entity, while using the plural puts a focus on them being two different people, but the Norwegian sentence above is so focused on unity already that I don't think it adds any such nuance. While I'd favour the singular in this case, I can't say that the plural sounds wrong to me.

It implies that they didn't.