Adding a minimal pyproject.toml silently breaks my build

[polortiz4]

I am trying to use pybind11 and scikit-build in a minimal repository based on their cpp example repo. When trying to build it via pip install -e ., pip claims that the file was "Successfully installed", but didn't actually build anything (I suspect it never even ran the setup function inside setup.py). I can confirm it doesn't work given the error message ImportError: cannot import name 'example' from 'hello' when attempting to use the module.

Interestingly enough, the project works when I either

1. run python setup.py develop directly (thus bypassing pyproject.toml, pressumably). For this, I installed setuptools, scikit-build, cmake, pybind11, and ninja to their appropriate version through conda install X.
2. Deleting pyproject.toml, relying on pre-installed build-time dependencies.

I don't see how my pyproject.toml file would be problematic. Can anyone help me figure out what's wrong?

My project layout is

hello-cpp
├── hello
|    └── __init__.py (empty)
|    └── hello.cpp
├── CMakeLists.txt
├── pyproject.toml
├── setup.py

setup.py:

from skbuild import setup

setup(
name="hello-cpp",
    version="1.2.3",
    description="a minimal example package (cpp version)",
    author="Me",
    license="MIT",
    packages=["hello"],
    python_requires=">=3.7",
)

CMakeLists.txt:

cmake_minimum_required(VERSION 3.4...3.22)

project(hello)

find_package(pybind11 CONFIG REQUIRED)

pybind11_add_module(example hello/hello.cpp)
install(TARGETS example LIBRARY DESTINATION hello)

pyproject.toml (I'll note that the problem persists even without constraining versions here):

[build-system]
requires = ["setuptools==65.6.3", 
      "scikit-build==0.15.0", 
      "cmake>=3.18", 
      "pybind11==2.9.1",
      "ninja"
]
build-backend = "setuptools.build_meta"

src/hello.cpp:

#include <pybind11/pybind11.h>

int add(int i, int j) {
    return i + j;
}

PYBIND11_MODULE(example, m) {
    m.doc() = "pybind11 example plugin"; // optional module docstring

    m.def("add", &add, "A function that adds two numbers");
}

I'm trying this in Ubuntu 20.