I did a google search for the tiebreak rules and it brought me to another boxing forum where a poster posted the following: Tiebreakers in case two or more boxers share the fourth semifinal berth after the three group stages. The first tiebreaker is head-to-head competition, the second is total knockouts, and the third is total judges’ points accumulated in the three fights.

Not sure if it's legit but it would appear to make sense.