well, if dust and dirt reached the air mass sensor, it certainly reached the engine, too! Where else would it go? The air mass sensor is not an air filter, and is downstream of the air filter. Once dirt gets past the air filter, its going into the engine. It seems an impossibility to argue that enough dirt got into the system to destroy the air mass sensor, but somehow NONE got into the engine. Your dealer is feeding you BS, IMO.
(whether it was enough to significantly wear or affect your engine is another issue, but its BS for him to say "no dust or sand reached the engine." I'd ask him to explain where it went, then!)