Does this just mean that the US secretly has much worse stuff?
Not necessarily. Chemical weapons have been off the table for a long time. They've got all the stigma of nukes but without MAD to justify them. Militaries tend to be conservative about weapons, though - they don't want to throw away a working weapon just in case it's needed.
My guess - the existing weapons are probably outdated and it makes little sense to develop new ones given that it's highly unlikely they'll be used. It's not like we fight the kind of wars where they'd be effective (think WWI here) anymore.
"Conventional" weapons can do more damage with more accuracy than chemical ones.
Aside from killing civilian populations, there's no reason to keep them around.
That said, we probably have some particularly nasty "conventional" weapons somewhere to accomplish the same outcome.
I mean, we have nuclear weapons
Chemical weapons have gone obsolete a while ago. We already know they've had far better stuff for a long time now.
“Declared” is doing a lot of work in that statement.