It would be interesting for them to stop smoking while all this was going on and see if there were still complaints of smelling it, while they weren't. Yeah, if you smoke I can usually smell it in your house or your car and sometimes on your hair or your clothes. In another apartment though...Unless something is wrong , that is pushing it IMO.
It's not as far-fetched as you might think. I live in a second-floor condo unit. The people in the unit directly below me are smokers. In one room of my unit (formerly my bedroom), the smell of smoke is very noticeable at times. Visitors have commented on it as well, and my clothes picked up the odor, too. So what I did was swap out my bedroom and office room. It's only the one room where the smoke odor was really noticeable.
I lived above a person that smoked years ago. I don't know how it came through, but it did. My apartment reeked.
I moved when my lease was up and didn't have the trouble anywhere else I ever lived, so maybe it was the structure?