HP India — rusting on motherboard mounting/grinding holes

I have a hp laptop model pavilion x360
S. No. 8cg60826pc (Under warranty) which is having some brightness issues. The brightness is stuck to high and i am not able to low down the brightness. I managed to get it to the service center located at noida sec-18. All they say is there is some rusting on mother board in the mounting holes where they put nuts and they cannot cover that issue under warranty. But whose issue is that i don't know since i have never taken to laptop to any local repair shop and they have asked me to pay rs.30000/- for a new motherboard. I don't know how hp is compromising with the quality. I am also attaching the motherboard pic. I won't recommend hp to anyone in future.