Translation:The relationship will be destroyed by that.
I don't think it is. I have the same thought on this exercise though. It and That /can/ refer to the same object in English, but generally 'it' is the abstract and 'that' refers to a specific object or concept.
Can any Danes gives us examples of how this sentence would be constructed in Danish to first refer to a 'that', and then again to refer to an 'it'?