Intar, it's a neutral way of speaking, when you write about something from a certain perspective (In this case it was about japanese vessels) you use that word. It does not mean any sort of support for the IJN, all it merely means is "Unfortunatly from their perspective".
Writing "Fortunately" would make sense if it was a story about Allies vessels, as in "Fortunatly the IJN vessles were sunk before they could cause any harm". However using that wording when the story is about IJN vessels make no sense, and imparts a clear historical bias from the point of the writer