It is highly unlikely that most people could use these two facts to narrome things down to even just a few people. How many of us know whether or not our neighbors are at workEven among co-workers, the number of those which you can say with certainty were or were not present at the time of a killing is minimal.
If we limit the search only to people with flexible schedules, outside jobs or absent from work at thoe specific times of each killing, we aren't really eliminating that many people.
The guy has shown he is meticulous in his planning. I am fairly certain he has his bases covered pretty well.