But I guess the main goal of the article is to show that, even though "radiation detector" sounds like a complicated thing to most people, or at least requiring some nontrivial components, it is actually possible to get started with DIY experimentation with very few and simple components and little effort. You might indeed get better results with ZnS:Ag phosphor sheets, but by showing that it is possible to build at least something with just a webcam that people already have you massively increase the likelihood that the reader actually starts to build and experiment. And when people already start experimenting, then many of these potential problems get incrementally noticed, figured out, and removed, and all kinds of cool projects can result. But yes, it would, indeed, have been nice if the article would have provided more information about testing the detector against various potential interferences and problems.
[0] http://spie.org/Publications/Proceedings/Paper/10.1117/12.20... (paywalled, though)