SCons is a volunteer-run open source project. As such, the SCons documentation isn't always completely up-to-date with all the available features - somehow it's almost harder to write high quality, easy to use documentation than it is to implement a feature in software. In other words, there may be a lot that SCons can do that isn't yet covered in this User's Guide.
Although this User's Guide may not be as complete as it could be, the development process does emphasize making sure that the SCons man page is kept up-to-date with new features. So if you're trying to figure out how to do something that SCons supports but can't find enough (or any) information here, it would be worth your while to look at the man page to see if the information is covered there. And if you do, maybe you'd even consider contributing a section to the User's Guide so the next person looking for that information won't have to go through the same thing...?