Winamp at this point in time is just a trademark that ended up at a Belgian company. And I think they want to use it to launch a music streaming service. The rest of Nullsoft went down with AOL. I wonder which party would even have the legal rights to move against such a source code leak.
They are claiming they have rights to the code too: "I am acting on behalf of AudioValley SA who currently owns the copyright to the Winamp code after a purchase from AOL."
NSIS has nothing to do with the lot that own Winamp & it was a clearly defined "not part of the sale" as it has been OSS from early on around it's v1.0 iirc which was a long long time before Winamp was eventually sold.
I still think they should be using their own forum with an archive of the NSIS sub-forum to ensure that there's a clear distinction between them & the lot that now own Winamp (as there's no benefit for them to be keeping that forum running other than a trickle of ad revenue) but that's down to those that lead NSIS to decide upon.