Hi Smilla,
The Peaceful Pill Handbook states the following (on page 76 of a 2014 PDF version I have):
My guess would be that if you were a heavy smoker, maybe your ability to fully exhale and inhale could be reduced? This is the only thing that really comes to my mind that could perhaps be the reason for those recommendations. I have heard smokers say that upon quiting, they just feel a lot better very quickly, even after days, so maybe it could make a meaningful difference.
Due to the nature of this method of exit, it would make sense to try to have as good lung function as possible in the moment of executing the method. So, if at all possible, maybe it wouldn't be unreasonable to stop smoking for a few days. But keep in mind this is all my non-medical speculation. :)