I don't think placebo effect applies - he's literally going through mental reps (probably doubling the number of times per week when he hears Kliff Kingsbury make a playcall, and then going through everything he has to do pre-snap). Just from a repetition standpoint, that's got to be invaluable. He probably knows this offense better than anyone other than Kingsbury himself and it's week 5. How much the actual 'throwing' aspect is helpful, no idea there - it's hard to know what he sees, how accurate the VR is in terms of improving his accuracy, etc... but the results seem to speak for themselves.
I read that they are actually throwing the next opponents defense up against him and programming the D to do the kinds of things they actually anticipate in the real game. That's crazy detail.
I suppose another way to put your point would be wondering if it's more the psychological edge the extra reps and familiarity with the offense gives him, vs. actually improving his accuracy. But then, how do we account for record-breaking accuracy be a rookie?