There seems to be a kind of informal agreement (somewhat backed up by the arms-reduction treaties) that the major nuclear powers won't secretly introduce new nu

There seems to be a kind of informal agreement (somewhat backed up by the arms-reduction treaties) that the major nuclear powers won't secretly introduce new nuclear delivery mechanisms, so therefore you can safely (if you put trust in this) assume that things not fitting into the known US/USSR/Chinese/etc. nuclear delivery methods aren't delivering nuclear weapons. Put differently, ICBMs officially can be nuclear weapons, and so basically can't be used for delivering conventional ordinance by the major powers, even where they would technically be a good choice.