There's no proof. All they've said so far seems to demonstrate a lack of understanding about what copyright actually is, as well as a general case of business immaturity.

Similarities in design doesn't warrant a DMCA takedown notice, period.

What he says sounds kind of reasonable.

But then when I see the side by sides that have been posted here, even the things that have been taken down, I get a big "fuck off" stuck in my throat.

Similar is not copy. Unless you want the same environment for copyright that we have right now with software patents.

This guy deserves the negative opinions - his DMCA request is completly invalid (he just gives the URL or their front page an no specific illustrations he claims are infringing), and the images that were removed from flat UI after the request are somewhat similar but not at all an infringement.

Unless he gives some solid proof, he does not deserve the Benoit of the doubt.