I think you are misunderstanding what happened. Their portfolio only had an unrealized loss, if they held they would have been whole. They were forced to sell

I think you are misunderstanding what happened. Their portfolio only had an unrealized loss, if they held they would have been whole. They were forced to sell long duration assets before they matured. This is the very definition of a duration mismatch. Short duration money got called back while the long duration money was below par. if they could hold the long duration to maturity they would be whole with no losses. Not sure what I'm failing to explain. Can you better explain why you disagree?