They likely mean some of the numerical packages that call out to BLAS/LAPACK or whatever as all that was originally written in Fortran. I think a lot has been converted to C now.

SciPy is a popular Python package which exposes functionality written in Fortran to Python, for example.

numpy / scipy both use fortran. Scipy has fortran as a build dependency (ie a large portion of it depends on fortran), and numpy can be built without fortran, but has large portions of optimizations in fortran.

anyone who's ever used numpy/scipy has been using python integrated with fortran.

I just recently made some Python wrappers for a Modern Fortran optimization library for derivative free optimization. https://github.com/libprima/prima

It basically had to go through Fortran -> C -> C++ (with pybind11) -> Python. At one point we had a slightly simpler setup with Fortran -> C -> Python (with ctypes), but ended up going with pybind11 after some discussion with the SciPy folks who steered us away from ctypes.

Not the OP, but I have written a fair few python wrappers around old Fortran code. There is a pretty great tool called f2py which automates almost all of the hard work. If the Fortran code isn't to weird it can completely autogenerate all the code you need to call your fortran function from python.

Fortran is actually pretty okay if you're doing matrix or vector operations! Builtin cross products, you choose how the array is indexed when you create it, can enforce functional purity to help you parallelize, etc

It is hot garbage for almost anything that isn't math though - which is okay, because it's been focused math from the start - strings and parsing an unstructured input is an exercise in pain for example. And the ecosystem is heavily geared toward math and scientific computing, so you will find yourself rolling your own stuff quite often if you deviate from that niche