I think there’s actually something slightly complicated here, because what Boccacio seems to be saying is that people may have been trying to protect themselves, but fundamentally these choices were the result of desperate helplessness, and were more to assuage the minds of the people who practiced them than to protect their health.

Firstly, as a textual matter, the whole point of this passage is to point out the helplessness of people against the disease (and excuse me for using quite a dated translation here, I don't have access to my copy right now). For example, as he begins discussing the disease, Boccaccio writes:

In Florence, despite all that human wisdom and forethought could devise to avert it, as the cleansing of the city from many impurities by officials appointed for the purpose, the refusal of entrance to all sick folk, and the adoption of many precautions for the preservation of health; despite also humble supplications addressed to God, and often repeated both in public procession and otherwise, by the devout; towards the beginning of the spring of the said year the doleful effects of the pestilence began to be horribly apparent by symptoms that shewed as if miraculous.

Boccaccio lists here all the potential precautions a city might be able to take against the spread of illness: Quarantine, cleaning of the city, prayer, and other methods (the last of which seems to suggest to me getting medical advice, but I'm not certain), and describes how they were all ineffective against the plague. He even goes so far as to describe the effects as miraculous, placing them firmly beyond human understanding or power to solve.

The implication of “miracle”, and the overall point of the passage, is that there wasn't anything anyone could do, and that people turned to desperation as a result of the effects of the plague. Boccacio later writes that the disease seemed to make a mockery of medicine, though he admits that he’s not sure if it’s because the disease was simply resistant to treatment or because there were many poorly trained doctors running about. Still, to me, given everything else he’s said and the general characteristics of this passage, that seems like a bit more of a literary flourish than a real question. It’s clear that Boccacio doesn’t believe anything could be done to protect against the disease.

Indeed, when he’s describing the reactions to the disease, the words he uses in the original Italian are “paure e imaginazioni,” “fears and imaginations.” The reactions, therefore, are not the result of rational knowledge of medical science, and therefore the belief that these remedies will work (and as he’s established, medical science was helpless anyway), but desperate fear for their lives. I don’t think, therefore, we’re supposed to take seriously the idea that drinking and partying would protect the people, but rather to understand this as a way people attempted to imagine they had control over their lives. Indeed, when he describes HOW these people indulged their appetites, Boccacio mentions that innkeepers basically didn’t give a damn if people drank them out of house and home, because they expected to die too. These aren’t people making rational calculations about the future, they’re helpless and desperate.

Boccacio then goes on to say that the most reasonable among the lot were those that chose to flee, because there was no medicine more effective than getting away from the disease, but after that, he goes on to say that, among all these groups, some died, some lived, ultimately affirming the message of helplessness. No matter what you did, you might die, and there wasn’t really anything that could be done about it.

Lastly, venturing beyond the text itself, Boccacio was an educated man, and seems to have been aware of the medical theory that dominated his time (he critiques doctors for not knowing anything about it, after all) and if so, he would have been away that in Galenic medicine, the balancing of the body was considered the best way to treat disease. I won’t go too far into the details (mostly because I’m no expert on it), but no doctor at the time would ever prescribe excessive drinking as a cure, and no educated person would think it would actually help. Boccacio seems to be a bit ambiguous about what people THOUGHT they were doing, for literary purposes, the whole point of the passage is that people were out of their minds with fear, but he seems to be saying that nothing anyone could do helped, and that they were more or less drowning their sorrows.