The theory is fine, although as the author stated there are a number of variables. What is not fine is that these companies have not shown how their products even work and are never subject to any scientific review. They may be measuring something entirely different for all we known.
Right - that's an issue with the implementation and the usage of the product. NOT with the science.
I agree with you on the breathalizer - I vehemently disagree with your ignorant anti-science inferences from it.