They're not claiming to get that many values per pixel, they're getting that many values overall for the medium through which light passes between the card and the phone. The idea light comes from a source (e.g. sun), bounces off the various colors of the card and thus produces hundreds of different spectra, those all pass through a medium, and land on the phone camera. So you're getting one measurement consisting of hundreds of RGB values that each represent intensity of different spectra, and you combine it all together to get a single spectrogram.