Read the original study paper (and not the media rant about it). A couple of scientists found a couple of half eaten white-beaked dolphins which had potentially drowned after being entrapped in ice (just before the bear got a dart shot into it for a capture-release program).
Polar bears eat various species of marine mammal, including larger whales, and are both hunters and opportunistic scavengers. This is a chance observance and not a story about extension of polar bear hunting behaviour.
