A Task runner (orchestrator) is a build tool that permits to run all kind of task (generally build operations) in serie or parallel.
A task runner works on a stream of file selected from the file system while a bundler will work on files selected from the dependency graph.
As they don't work on a dependency graph, they don't support dependency graph related task such as:
They can still perform most of the build operations such as transpiling, minification, bundling.