Is Flat UI a ripoff? Yes, sure, and they come across as more than a bit sleazy. But are they in clear violation of LV's copyright? Absolutely not. In design as well as in pretty much every other human endeavor, all things are derivative. That's a core truth about technology and civilization in general. The Flat UI dude did not straight-up copy LV's stuff though. Still, he re-implemented it and he deserves to be called out for it. He should at least have given LV some credit for inspiring him.
That's where it ends though. The rest is all on LV. That DMCA abuse was stupid and unnecessary. Sure, you can blame HN for making that observation but that doesn't change the fact that it's not the observers fault you screwed up. Failing to admit that (and apparently making DMCA jokes on Twitter in bad taste) only serves to emphasize why people are upset with you.
LV, you used a bad law in questionable circumstances to shut up something that was no threat to you. Then you doubled down, smugly and with not a small amount of self-righteousness, when people called you out on your ethics.
The saddest part of this is that you could have easily turned this into an ad for your stuff. If you simply had come out and graciously acknowledged the similarities between Flat UI and your icons, people would have sided with you. Your implicit counter-argument that it doesn't matter if you're acting badly as long as it's the other guy who broke the law is interesting, but it's also far from certain things will actually turn out this way in a court of law.
This was a failure on many, many levels. Doubling down on bullying while at the same time crying "not fair!" is just one of them.