It's certainly not the intended behaviour, and it's not what I see if I run docgen on a package.
The --start-page option will let you control where it starts, but that should be set automatically if you run it on the root of a package. But if PROJECTLIBS isn't a directory with a pubspec in it, then it won't think it's documenting a package, so you might need that.
You will likely get parts of dart:core generated, because even if a library is not included, if there are references to classes in a dependency it will generate the data for that class. But it shouldn't be the whole thing and it shouldn't start there.