Undetectable sometimes means less than 200 or less than 50 viral copies per ml (depending on device sensitivity). In contrast the peak in the acute phase is 10^7 copies per ml (and settles to around 10^5 in the chronic phase), the probability of transmission is infinitesimal. There are many studies following very high risk cohorts (sex workers, intravenous drug users, etc) that have been able to empirically confirm this.