Would healthcare costs increase (in the long term) if all smokers quit?
It is a reputable journal and the results are surely correct on a "total dollars" sense. I guess the point they are making is that because health care expenditure of old age individuals is very likely to be high, there is some 'social benefit' of dying before old age. 

However, it seems somewhat meaningless: 

A) First, by this logic, murders also decrease health care costs. We're not trying to keep smokers form smoking solely because they will be a health care burden. We also want these people to live long, rewarding lives.

B) People contribute to society, their families, their friends and so on. They also save and, specifically, they pay health insurance premiums while they are alive. So, while they might require more care in the long run, non-smokers might well be able to pay or it with the taxes on their income and the productivity they bring to society. Similarly, we all invest through our taxes, on public education, and it goes to waste when somebody that has received public education dies.

C) There are enormous variations in the amount of healthcare spending that old people receive across countries, so the estimates of how much health are old people will receive has got to be very imprecise. 

D) Investments pay a positive rate of return, so if we saved the money now we could pay for a lot more care later. Put differently, all the resources that would not be devoted to the care of smokers now would be used to improve our lives meaningfully, potentially even helping us do research into how to take care of old people effectively, which would again reduce the future expected cost of caring for the smokers.