Okay, I am an amateur mechanic, and I do almost all of my own work. I have no idea if I am a good mechanic. I guess I'm good enough that I have no complaints about any of my bikes. However, bicycles are forgiving machines, so sometimes "close enough" works.
The OP has stated that the LBS has replaced the cassette under warranty, and replaced the chain at no charge. I see no possibility that the LBS is running some kind of scam on him. Business people would rather be selling bikes and selling services, not fighting with the supply chain over warranty claims. I assume his mechanic has consulted with his colleagues about this, as we all agree the stated facts are unusual. We are all trying to second-guess the work of professionals who have seen the problem up close and personal.
I personally have zero confidence that I could look at a cassette, and determine if it was worn out. I have less than zero confidence that I could look at a picture of a cassette, and come to any conclusions. As others have suggested, I would need to have an issue with the drive train (poor shifting, ghost shifting, noise, etc.) before it would occur to me to diagnose a problem on a nearly-new bike.
I am also going to rule out the possibility that the OP took the bike to a car wash, and blasted off all the lube with a high-pressure spray. My arm-chair SWAG (Scientific Wild-A**ed Guess) is that there could be a chain line issue. If the OP types in a query in the ? box, he will get detailed instructions on how to measure chain line. Failing a precise measurement, shift the bike to the suspect cogs, and observe the chain line from above. Is the chain wildly skewed when the drive train is in the suspect gear? How skewed is it in a middle gear?
Was the original chain measured for wear? I would expect a chain to wear out before a cassette. If the old chain is still available, I would examine it for unusual wear and/or chain stretch.
My $.02. I'm looking forward to the OPs next post on the creaking BB, so I can pontificate on the evils of PF BBs. And, if his drivetrain is a 1X, I can grouse about that, too.