Both are analogies and therefore imperfect but the oxygen/hydrogen one is closer to the mark than the batter ingredients one because it is dealing with stable chemical bonds. An amalgam is not just a cake mix. The "ingredients" are chemically bonded to one another.
Quackwatch is trying to stop unscrupulous people from taking advantage of uninformed people through scare tactics and "smoke and mirrors" devices such as that meter shown in the article.
If you feel better without them in your mouth and you have the extra cash to drop on that and don't mind the pain involved, fine, that is your decision.
I got all of mine taken out for cosmetic reasons. I didn't feel any different. A dentist I know who specializes in doing this kind of work (and has gotten very rich doing it) tried to convince me that the fillings were causing my seizure disorder. Nope. It didn't go away.