I've followed the Half Life community for ages now, and something extremely similar (aside from possible RCE) happened a few years back. You CAN actually gain access to the source code of source engine, as well as Half Life. But it's extremely expensive, and valve obviously has to approve it. That's how that failed abortion that was Hunt Down The Freeman came out.
So, a few summers ago someone in the HL2 modding community started leaking unused assets as well as beta assets which is very illegal because he had acquired HL2, and the dude hasn't posted shit since. Like he just went into hiding. Valve doesn't play any games, so if whoever leaked this is found lmao enjoy prison.
But I wonder if the two are related, because the the CS leak is from like 2017-2018 and that was around the time the HL stuff was happening.