The artist probably should have handled things a bit better than that, however, there was nothing wrong with the files. The problem was, in fact, on your end. Asking for a refund right away is a bit like going to a restaurant, ordering a steak, eating all of it, and then demanding your money back since you didn't like it.
Keep in mind, that there are scammers out there that download files, complain to the artist and support that they either downloaded the wrong format, or the files do not work, to try and get a refund of their money. They get to keep the good files and go off and do the same thing to another seller.
Just because the model you purchased doesn't fit on your printer now, it doesn't mean that it won't fit on a larger printer when you upgrade or be able to be cut up with a properly configured PC. Quality models have more polygons and thusly will require more system resources to cut or print. The artist can't subdivide their models for every type of printer out there. It is up to the buyer to figure out how to subdivide the model in order to print it out on their particular printer. Just because you can't do it with your current setup and resources, does not make it someone else's responsibility to figure out how to make it fit on your printer. There are also customers who do not have their own printers and get 3rd parties to print stuff out for them.
The artist does seem to be a bit insensitive with his remark, however, there are always two sides to every story and you omitted your own direct quotes from the conversation. Context is important in every conversation and the artist was not wrong in what he said, but perhaps the way he said it.
The artist is under no obligation to refund your money, since he obviously knew the files were good, and other customers had no issue with them. You could have complained to CGTrader support, but they would have tested the files and came to the same conclusion. If the situation was reversed, and your were selling something that you know works, and a customer complains that it doesn't and demands their money back right away, you may have the same reaction.