First of all, they’ve announced they have release the source for the game engine used to create Serious Sam – not that they have released the source to the game itself.
I had a quick browse of the released code and this is what I found:
1. It’s the source code only. There are things like level editors included and although there is the source to a program called SeriousSam, it’s not clear whether this is the actual game or just a test app.
2. It’s doesn’t include the resources associated with the game (3D models, textures, etc.). There is an example set of resources but that is not the game itself. There’s a comment on github about copying the resources from the game reinforcing this.
3. It’s very much a Windows application. It assumes it’s running on Windows, was compiled using Microsoft development tools and relies on the Windows libraries.
I’d say the chances of it being ported to work on the Pi are negligible.