I also do not buy that garlic boosts the immune system. As stated before I have never seen anything to support this, just a regurgitated comparison to Vitamin C.
There is evidence that garlic does help treat white spot (not ich in saltwater). Some people have used either freshly pressed garlic of New Life Spectrum's Thera+A formula (seems to be the only food that has enough garlic preserved well enough to treat parasites). The lack of undeniable scientific support (because there is so much in our hobby that has been supported by scientific studies) leads many people to make the comparison that it just boosts the fish's immune system (without explanation) but doesn't directly kill the parasites. In those cases people cite the lack of evidence to discredit the idea that garlic actively kills parasites.
And again, it is not just any garlic. In general only freshly pressed garlic has the chemical that will kill parasites. So all those foods that have garlic added to them and all those liquid additives you can buy at the LFS
are not going to do it. This is why there are so few cases demonstrating it, almost no one is using the right type of garlic. I have talked to a couple people who have treated white spot in reefs using only garlic.
Yes, the idea is that if the fish ingests the right form of garlic it will be passed on to parasites.
At the very least this evidence shows that garlic can kill parasites, something many say is not the case or impossible. So at the very least it has the potential to help us treat white spot and other diseases and is a good option to consider when dealing with parasites in a reef.
It won't be 'common knowledge' until it becomes common knowledge. These are recent studies that are only supported by anecdotal examples in the hobby, again because the right forms of garlic are not being used. The more people say 'I used NLS Thera+A/freshly pressed garlic to treat the ich/white spot my discus/tang had' the more people hop on regurgitating "no, it went away on its own, there is nothing showing that garlic can do this, it is just coincidence, read this article that has poorly analyzed references that 'proves' my opinion", this only continues the misunderstanding of exactly that garlic can and cannot do.