No.

This is directly using (even hotlinking some assets) the OPs copyrighted materials.

FlatUI had 3 icons that were "similar"[1] to icons in LayerVault. There wasn't any clear copying of assets.

[1] for some values of similar, including "not similar".

FlatUI was inspired by another website. In this case, both websites seem to have copied all the assets up to the javascript from gocardless since they received sentry notifications.

If they do the same design, but using different bits, they're not copy. But yes, they copied the design, files and everything, this changes the rules of the game. Now you can sue them for that, but you cannot stop people of using the typography, colors and layout of any design, that's called style.

It's silly the ripp-off, made me laugh.

Not sure if you read the description provided. They ripped the source code as well, not just the design. That goes under copyright, as would copying someones graphic assets. If it were using similar graphics or design, then yes, it isn't a copyright issue.

Why? They are stealing OP's copyrighted works and posting them on their website.