Its about as clear as mud!
Took a look last night and it left me wanting to call them as their site doesn't actually state what they will accept.
Speaking as an ex-baggage handler. I'd want the bike packed in something that meant I didn't get hurt handling it, nor could me handling it damage it. Luggage transfer isn't the easy, smooth process you're shown on TV. Time constraints mean they'll want it all loaded as quick as you're able.
Done it a few times after reading that policy of their. No issue at acceptance and no issue at arrival. Agree on the rough handling thus the padding and bubble wrap where needed. Also used the Fragile and this side up stickers when I used the cardboard box.