It was actually down at the time of that post too. I know because I was using Flat UI in a new project and had to ditch their vector illustrations which were the items in question from this DMCA.
I was not aware from the original post that the project had actually been taken down, and now thanks to pyre's comment I know a bit more about how the DMCA takedown process works. It seems to me that the process is quite vulnerable to Denial-of-service attacks, as demonstrated by the case in question.
They took it down, but you used to get a 404 when you visited the repo. This is the first time that it explicitly says that a DMCA request was executed.