that is actually incorrect info the stanley Parable was made in the portal 2 engine but the stanley parable Ultra Deluxe was made in Unity so i see why you might have thought that
Oh yeah sorry about that, it was the first thing that came up on google lol.
After doing some more research I found out that Stanley Parable was made by cloning the Source 2013 repo and following the valve wiki. Though i still don't reccomend this. But do what you do ig.