I think this was a huge blunder on their part. Huge. I will never order any of their product. All manufacturers have QC issues from time to time. When a customer brings those issues to your attention, no matter how snarky the tone, you do NOT take them to task for their attitude. The first thing you do is you ship them the match grade product they paid for. Then you take ownership of your poor QC, you don't make excuses for it. You thank the customer for giving you an opportunity to grow. I don't even understand how there was a question of whether or not they should respond to his email. Hell yes you respond. You screwed up, and you owe the customer an apology and the product they paid for.
