Apple care is for manufacturing defects so what they're saying is that its user damage not a defect. Tbh, I can't tell from the picture if its pressure damage or something else.
Do you hold the laptop oddly or drop something on it?
I know a lot of people with unibody MacBook Pros. Some people have these pressure marks, others don't.
The people who have these pressure marks are the people who carry their laptop in one hand like this. The entire weight of the laptop is being balanced by the tiny bit of aluminium under your thumb. Over time, if you hold your MacBook in the same place, you'll get pressure marks.
I held my old MacBook pro like this. It developed a mark like this exactly where my thumb held it.
I put 2 and 2 together, and I've never held my new MacBook Pro like this. No pressure mark.