Is "keyboard paint wear on screen" a real thing?
I would say three things.


The most likely cause of any discoloration would be hand grease that transfers via physical contact. That can happen when you have it closed and something is weighted (book or hand) holding the screen glass on the keyboard. Backpacks and briefcases also are excellent ways to compress the gap between glass and keyboards.
Wear is more likely over many commute sessions than one time “hot” contact, but I probability wouldn’t tell Your buyer they’re “dead wrong” if you want them to negotiate.
There is an anti glare coating that will wear due to cleaning with solvents and not water as well as with wear over time.


Totally your call on if you cleaned it - some people can’t care less about minor scratches and imperfections - others feel they need a brand new machine when they buy used. I have seen this to be very excessive where the $150 is basically you splitting the $300 repair to get new glass installed and others where you can’t even tell this unless you have special lighting, look carefully from an off angle.

Taking good cell phone photos of the display when lit and black would be a good thing before you send out the mac for inspection if you think the buyer might care about this. Also, you might ask the buyer - hey, how hot do you think the glass got for this? The physics here has nothing to do with heat and all to do with bending and friction and wear patterns.