I'm not sure if we can actually localise particular EEG's that precisely. In any case, that just punts on the problem - subject reports they did or did not which we use to make an assessment. We still rely on subjectivity.

Like, I really don't see any way of avoiding this in psychology and neuroscience (and indeed even if possible, it might not be a good idea).

But we can localize it that precisely, and cheaply. In that particular experiment, they found that the areas of the brain that are commonly associated with raw visual input activates precisely whenever an image appears, subliminal or not. Then a large, delayed negative wave reliably appears in the frontal lobe only when the patient affirms that they saw the image. It's using objective methods to confirm the subjective.

These sorts of experiments are helping with making prognoses of whether a patient recovering from a coma will remain in a vegetative state, or if they'll regain consciousness in time.

Do you happen to have a paper link, rather than a book link?

Sure, I don't have the book on me, but this journal entry (written by the author) in Neuron outlines the main points raised here with plenty of references/citations to back it up. https://www.sciencedirect.com/science/article/pii/S089662731...