He also made his first code contribution recently (a far from trivial one):
I would normally wait for other code contributions before nominating someone to the GitHub org, but given that Benjamin has a great track record of user support and training contributions before that, I think it would be really useful to have their expertise in PR reviews, and ease his further code contributions.
Adding him to the Developers team would let him label issues, run the CI pipeline on their PRs automatically, and also review other pull requests.
As you said, that first code contribution was far from trivial. I think the ability to find and resolve that issue demonstrates a firm grasp of the codebase and a sound technical knowledge. I support adding Benjamin to the GitHub org!