I had a similar experience with an authorised repair place - my keyboard went on the blink (as has been reportedly recently with all the touchbar MBPs. Upon opening it they found a coffee stain from where I had spilt coffee on it months before. Their response "we can't fix it sorry, Apple wouldn't allow it"...
Very confused about my situation, I contacted apple on chat again, and told them about my experience at the store. The chat person called their manager, who again threatened me saying that if I want to challenge their diagnostic test I can, otherwise I should stop contacting them. I have no knowledge about how to find out about this test, what it is, does it consider normal usage conditions of laptops or anything like that. If anyone has had similar experience or knows about this diagnostic test do share, I don't want to pay 700$ extra on a macbook