Don’t kid yourself. He could very well be sick with ebola while the apartment residents are not.
There are many diseases and examples where some, but not all or maybe only a few of those in contact with infected patients become ill while others show no effects just as some of those infected may survive while many of the infected do not.
You are correct that it is possible. The infectious dose of the virus is very small and he could have had a paper cut or other entry point while they did not. It is not likely, but possible that he got it while they did not (yet).
There is much they don’t understand about virus transmission.