- It's a land unit, it occupies tiles like other land units and embarks in the ocean and lakes
- It's an air unit, it can hover over mountains
- It's an icebreaker, it can move through the ice
Helicopters have always been iffy about what they are, but them being able to sail through the ice when embarked can only be an oversight in the "Ignores terrain movement cost" ability.
If it can't be classified as a bug, please move this thread to general discussion so people are made aware of the issue and can judge the oversight appropriately. I'm kinda curious about how people would classify it, come to think of it.