I'd say it's all those things, likely in different combinations based on the circumstances. Cook's notes indicate that he may have considered it a panacea of sorts, triggering a bunch of different stuff that helped overall health, but keep in mind this was something to tune of 150 years ago, so the information he was working with may have limited the scope of his understanding. For all his otherwise infamous reputation, his work aboard the Belgica was nothing short of pioneering for the time. His life after that expedition over-shadows any positive contributions to science he made, unfortunately.