I have heard this before, and there’s no way in hell I’d be caught kissing that thing. Euch! There must be some bacteria, virii(??), etc on it, especially in busy periods, where there’s a constant stream of people. Yuk.
From when your body acquires the herpes simplex virus to when symptoms appear (ie, a cold sore) is certainly more than overnight. If that’s his story, it sounds like a lot of (wait for it) blarney to me. It would take more than a few days for symptoms to appear as a cold sore.