Information about nail object in tyre

So recently, while driving on the main road, a galvanize nail pierced my left rare tyre. To my surprise it did not deflate instantly but I was scared that I would have to buy a new tyre ($$$). So I went to my tyre shop and asked them. I got back some pretty good information I thought I'd share with you all.

Apparently, as long as the object has not pierced the wall of the tyre, you don't need to change your tyre. I can see the logic in this but I cannot explain it. So in my case, I just had to get a patch and I'm up and running. I guess FireStone tyres ain't that bad after all.