Bazel splits the build into multiple phases. Starlark only comes into play in the first two, load and analysis. During these phases, Starlark code doesn't have access to the filesystem, except in a few very limited cases like using the glob() function to expand a wildcard to a list of source files. Furthermore, it only generates an abstract graph of build actions. The Bazel engine is responsible for executing this graph in later stages, which might result in non-hermetic things happening but usually not.
Starlark has intentionally limited functionality such as lacking Turing completeness or global variables. This provides guarantees that it can be executed in parallel and will have a finite runtime.