Thomas is well know as one of the maintainer of IPython and Jupyter, and developed flit while working on the pep for pyproject.toml and the pip backend allowing things like python -m build.

Though `python -m build` only works _if_ you use something like flit or setup.py in the backend to do build the package and hence why you can set flit as a build-backend.

So yes, flit is one of the latest tool, and yes it is one of the things that push for the ability to use pyproject.toml+python3 -m build, you just seem to miss some subtleties of the toolchain.