The case law on butterfly knives ( there are 5 different ones in addition to the one you mentioned) all focused on how there is an additional step needed for the blade to lock open. The opening of the knife itself has been always viewed as meeting the definition of a gravity knife, but the fact it does not lock open without additional action on the part of the user has allowed judges ( who think correctly in my opinion) to dismiss butterfly knives as not meeting the full definition.