I've had my indoor cat Miss Dexter for about 18 months now and in the first 6 months she was very timid with visitors. She would hide and not come out at all. Over time she's got better and now comes out and sniffs at people and just happily sits and watches people.
That is all visitors except both mine and my partners Mothers.
All friends are accepted, but even now she won't come out at all if it's a "mother" visiting.

We can't quite work it out as neither Mother is noisy or loud. The only common factor is that both Mothers are smokers whereas no other visitor is. But neither Mother smokes in the flat, and I wouldn't say either Mother would really smell of smoke.

Would a cat really be that sensitive to the smell of cigarette smoke?

Cats have a very keen sense of smell, just because you can't smell it doesn't mean your cat can't.

Your cat will smell the smoke on their clothes, body and in their hair. My cat goes crazy when a smoker even just knocks on my front door as one visitor (who is a smoker) will usually bring my cat some pieces of chicken. Therefore cigarette smoke=food.