The date on the files in web file directory listings for this leak, are 2013. That's the archive itself as posted to the web. I have no idea what the dates are inside the archive, as I've not obtained it to look.
Let's also be clear about how easy it was to find. It was named 'amiga os source code 3.1.tar.bz2'.
Simple logic dictates that if they really cared they could have googled and found it, as many of us did. Much much earlier than this conference tweet "leak" announcement. But no, the it must be protected argument is weak because it provably has not been. That the web file directory in question has only now been taken offline in reaction to this tweet, is irrelevant to the long term previous easily discovered availability.