I don't think you can say that. What they've done is declared this particular knife is illegal because it is a dagger. I don't think that declaration implies that any knife that doesn't meet the same description is legal.having a folding knife without a guard that meets the lenghth requirement and is double edged is perfectly legal????
I think this just provides one more addition to the list of things that a dagger can be. But I think the list still only includes the items but is not limited to the items. I.e. it MEETS the definition of a dagger, but does not exclusively define what a dagger is.
It would appear that a single-edged blade doesn't qualify, but that's about all I'm sure of.
BTW, I don't think that a switchblade is illegal to OWN, anymore than a dagger is illegal to own. You just can't carry one in public.