Actually, user contributions are licensed under a CC-BY-NC-SA license under the terms of service. This is compatible with but much more restrictive than the terms of an MIT license, and is not free/open-source. (However, snippets of code ≲ 15 lines are typically considered not copyrightable; see also the discussion here.)
That being said, I think that this is less a question of legality than one of basic courtesy. I completely agree that it would have been more respectful to check with you when creating a package (or a pull request to an existing package) based on the responses to your query.
I would encourage you to try contacting the person amicably in case this was an oversight and see if you can resolve the situation in a more satisfactory way. If you have trouble making things right and want a neutral third party to talk with both of you, feel free to contact the Julia Stewards.