I had a tire go flat on my fiver just as I was backing it into my drive. I took the tire to the nearest tire shop and the guy tried plugging it. After three plugs he gave up (I noted it looked like he had to force the tool thru the tire when he found a leak). He then put it in a water bath and found it leaking in numeous spots. He wasnt a Goodyear dealer and the tire was relatively new so I took it to the Goodyear store. As soon as he looked at it he said the tread was seperating and showed me how he could tell just by looking at it. (GY replaced the tire under warranty with wear charge)
A good tire will be flat across the tread if properly inflated. This tire had a curved surface as a result of tread seperation. ( imagine your hand held out flat, palm down....that is good. Now slightly cup your hand and note how the top of your hand is curved across your knuckles....that is what tread seperation looks like). This caused many many air leaks thru pin holes in the tire tread which is what the original guy was trying to patch.
So was the guy scamming you? Could be. Could an experienced tire guy easily see you had tread seperation? Definitely! JMHO