Can't speak to candidates from the US but as far as I know, for international students, if there is a current student at HBS from your country, they call you to congratulate you. However, no idea on the timing of that call. I presume it's after the decision is available online.
I feel as though they wouldn't reveal anything before noon ET so that everyone has their decision available to them at the same time to avoid any issues (this is also why I'm not convinced by the mailtester thing but who knows)