They line speared it first. The traditional way is repeated spearing afterwords. The 50 was used instead of the repeated spearing, because it's much quicker. They didn't finish and retrieve the whale, because the coast guard interfered. Notice the whale died shortly after the coast guard came.
I think you are correct. This is the usual bs from the media. They probably harpooned it and then tried to use a single shot 50 BMG for the final “put it out of it’s misery” shot.
The media heard or asked what BMG stood for and then had their headline.