Air is supposed to enter the engine via the AMM (or be found via the MAP sensor in cases). The computer knows how much air is available and corrects fuel delivery to suit. (ther are other factors in that concern that I'm skipping due to relivance)

With a leaking manifold on an AMM equiped vehicle, the engine is only adding fuel (this is VERY over simplified) for the amount of air it's reading that the engine is getting. The worse the leak, the less the percentage is being read by the AMM and the leaner the engine (still leaving out many many details and factors here). In extreame cases, and with other concerns, it can get lean enough to cause detonation and cause damage.

OK, I hope you get that I over simplified this to an extreame measure. This isn't something that's "GOING" to happen. In fact it's not all that common, but does happen.

Any ASE (or such) qualified tech with more written education in auto mechanics than real life experience or true knowledge of operations of different aspects of a vehicle, will go by the book.......... IE, by the book, intake leak = engine damage.

Hope this gives you some idea of where your mechanic's coming from. Beyond that, I can't offer much.

Happy Brickslinging,
Frank