The claim that there will be “mass hysteria” is, of course, unfalsifiable (until we actually do encounter space aliens). More important, and probably what aliens should mostly concern themselves with, is how the world’s governments will react. “Hi – just wanted you all to know that we exist, but you are not ready to know that we exist, so please don’t tell anything. The longer these claims of – we’re just not ready – go on, the harder they become to sustain. This is probably the same threshold as just letting the world know they exist.