It really could have been anyone, and it could have been multiple people, different person/persons for each of them since they didn't arrive home at the same time. What if the cleaning lady or plant-person or personal-trainer or relative or friend had been the last to see H at the home earlier before B arrived home? I'm also assuming that there may have been others at the Apo offices where B was, after the time when said architect left.
I think that often the public may associate the 'last person to see a victim alive' with potentially being the one to have caused harm to the victim, since obviously whoever saw the victim alive last is a major clue in what happened in any given case. It seems that often LE like to keep that detail to themselves for investigative reasons. If media had pursued 'the architect' for comment, they may not have known which person(s) at the given firm was the one to have actually met with them.
I followed a case awhile back where a man killed his ex, whom he had 'a great relationship' with. LE kept it to themselves who was last to have seen the victim. Sleuthers spent lots of time theorizing who it was. Then later it came out that it was this wonderful ex, and their thoughts on the ex started to change. I'm sure police don't wish that sort of thing on anyone (since they may have had nothing to do with it), and people likely don't want to bring that sort of thing upon themselves either. So imo, I don't think it is that bizarre that we don't know who the last person to see each of them was, why do you suspect this is meaningful that it isn't known?