The argument is that exposure to mercury can cause a number of toxic effects, based on degree of exposure and individual susceptibility, that can lead to or exacerbate candida.
Nobody is saying it is the only cause of candida.
Btw, removing Amalgams is usually only the first step, though you wouldn't know that by reading Huggins. Dramatic immediate improvements are rare, and I think have more to do with removing the metal in the mouth than anything. Chelation is the key to serious improvements over time, though you shouldn't use EDTA, as was mentioned in another post.