As it stands, his accusation feels legitimate to me, but it's difficult to make an informed judgement without knowing who the vendor was and how Mark's mic was positioned (could changes in the position of people around him or the means he used to conceal it have led to the 2dB discrepancy, for example?).
The vendor was a guy selling $17,000 power cords. And it's amazing how the errors in level measurement from an actual expert correlated perfectly with price.
The excuse-making for frauds always amazes and saddens me.
edit: FWIW, I've tested and used phone-based SPL meters. The ones I reviewed were quite reliable. I wouldn't count on it for, say, a 0.1 dB difference, but 2 dB is well within the range of trust.