Yeah, that was as far as I got, as well, which is why I think it's some variety of websitefail.

There's no legit reason I can think of to allow motorbikes but not pushbikes: I've ridden the bike onto various ferries in various parts of the world alongside the motorbikes (Norfolkline queue two-wheelers up ahead of the cars and let you on first; Spirit of Tasmania don't bother) and it's never been a problem. Next time we're actually thinking of going over I'll look into it.