The World Boxing Association's Referee Manual makes it clear that the boundary between a legitimate blow and a low blow "is defined as the imaginary horizontal line through the navel to the top of the hips"
https://www.wbaboxing.com/officialarticles/wba-referee-manual
The manual goes on to state: "This is so regardless of where the boxer places his trunks and or protector cup. The Referee should establish the waistline at center ring in his final instructions".
Therefore the referee should have indicated prior to the fight where he regarded the boundary to be, especially with the ridiculous high waistband on the boxer's shorts:
https://i.guim.co.uk/img/media/6534ea8f697f4496bb202b40c8f93acddc34acc5/434_942_2966_1780/master/2966.jpg?width=620&dpr=1&s=none
He will then have to have kept that imaginary line in his mind when determining his decisions, which can't have been easy when he couldn't see the boxer's navel.
Some observers of the fight though have suggested that the referee didn't need to consider the boundary line, as (in their opinion) the sound of the blow striking the abdominal protector was clearly heard and thus firm evidence of a low blow.