Well, you can check the scripts of the tool yourself. They’re all in the zip file you download from nexus. It just sends the credentials to the Steam API and uses the auth token for the rest. That’s exactly what the Steam app on Android does. And it’s also not some tool from a random dude, it’s the official downgrader tool from the FOLON team. They even link it on their homepage. I get being paranoid with credentials, but in this case i saw no problem. Sorry for my rambling, but this “issue” is blown way out of proportion imo.

Ah, I didn't think about who made it. I just saw two options, either I go through the code to make sure it's nothing sus or I do it manually and because I didn't feel like going through code I did it manually. Should've paid more attention I guess.

As for the guide, I haven't tried the Heroic launcher. I generally use Lutris but I will try that when the big patch comes out. Hopefully they also upgrade to a newer version of Fo4 because the ultrawidescreen mod is having some issues.