[beseku]

"... they even managed to kinda lift the old DN icon ..."

I think you misunderstand. I'm not arguing as to whether the Flat UI guys copied them or not, but like the larger argument, the icon he is putting forward as copied work is itself conceptually very similar to other existing icon work(s). As a designer I would be more than a little miffed if that design appeared if I had created the Noun Project's version and not openly licensed it.

If that is the case, it somewhat undermines the claim that the assets are fully owned by them or even that ownership can be so quickly claimed.

[hkolek]

Similarity is completely irrelevant. If we independently arrive at the same original work then that is not copyright violation. That's not what copyright is about. We then each hold the copyright of our respective works, even if they're completely identical (in practice it will probably be hard to proof that you didn't just copy the other work if they're really identical).

IANAL, but that's what I was told by a law student.