Editorial Comment: 

It is difficult to make an absolutely accurate diagnosis from seeing a photo, but I will try to make some sense of this.

To start, the lines which are marked in the photo appear to be too smooth and continuous, to be simple cracks. The top one almost seems to me to be like a cut, while the others are less pronounced.

If this were a separation of different layers of the tire's construction caused by a poor bonding between the layers, I'd expect to see one single line where you could actually distinguish one layer from the other

The way in which tires are made makes it unlikely that a number of lines of this nature would appear in this area of the tire, especially if they appeared after the tire had been put into use. If you ever have a chance to visit a tire factory, you could see this yourself.

At this point I'd tend to agree with the dealer's assessment that this was damage cause by some kind of mechanical action of something external rubbing or pressing on the tire as it rotated.

I know it's not a pleasant surprize to have an almost new tire damaged but if the damage was entirely superficial, and non of the tire's cords were damaged I'd suggest that you still might be able to use this tire with caution -- for example keep it to be used as a spare for short-term temporary use.

TG