The entire facts are plainly that, a guy from a community called games.debugging, that crawls and decompiles alpha builds of console game, whose name was
zecoxao found 5 stable validation builds which QA approved builds indexed on a torrenting website (this means that your torrenting software uploaded the torrent to a database in hopes of sharing it around, its an automatic setting in some torrenters).
The guy tried to download them all but was only able to download one of them, specifically stable_validation_b49ee388, he looked it up at a glance, saw it had AES128 (basically, it uses a 128 character key that is used to decrypt it, without it it's virtually unbreakable) encryption and extracted the .pbd file and left it at that since he didn't care and it was tightly encrypted.
He took this .pdb file and shared it around his community as a neat thing, a russian kid (not kidding) who was in the group directly asked him what it was about, and he told him that it most likely was for Stalker 2, upon seeing this the kid immediatly went and posted about it to a VK group whose name is "Alpha-Beta-Build Maniacs", which got almost no attention but started a rumour. While the .pdb file is being spread around, Antischnapps, notorious russian sloptubber, goes and asks the kid where he got it from which leads him to
zecoxao and directly to the leaked build, Antischnapps being a big russian youtuber, spread it around giving it loads of traction, however, he didn't post a link to it in his VK up until it is shared around RedPanda's discord server, a big place for the SoC-to-UE project and general X-Ray Modding.
Vestnik, without any of the knowledge of where it comes from, and how it got here, grabs Antischnapps folder and boosts it as "one of his sources", this is a 200gb block of encrypted files that gets passed around for days without GSC worrying too much, because AES128 is unbreakable without having the key
Misery however struck for GSC, because in a hilarious blindspot, the AES key was logged in the .pdb, probably when they were troubleshooting their encryption systems, which leads to the entire files being decrypted, here is one of the guys from the games.debugging group explaining it all.
Here is my wild speculation theory: GSC were not hacked, neither were they dumb enuff to unwittingly let DHT on, a disgruntled emploeye who was discontent with the companies actions and the development of Stalker 2 purposefully grabbed all he could and indexed a raw .rar file to try and leak these files. Why? This has only come out recently but the development of Stalker 2 was a complete shitshow of mismanagement which is why it feels halfbaked, because they were running like headless chicken up until like 2020-2021 and then rushed to the finish line as soon as they possibly could, cutting and axing a lot of their planned content.