It’s possible they found a marker and correctly interpret that as some oxidation, how much would never be know on the inner parts except with a costly breakdown, which you would have to pay for. That’s why they have the markers, to save work that you won’t want to pay for.
If you have your phone back compare to one image below. If the marker is white then ask how they decided it was oxidised. Note Fairphone only report what Cordon tell them. So try and put the request in polite terms even if you feel upset and angry.
See this post for the user available ‘marker’ and the one two posts above for the other two.
