As stated clearly in their CONTRIBUTING.md, this project follows the very common practice of requiring prior discussion on an issue, before submitting a pull request.
In this case, the pull request author completely ignored that process and submitted a 400+ line unsolicited pull request with no prior discussion. All of the wasted effort would have been completely avoided if the PR author bothered to post a single-sentence issue asking if the maintainers actually want this.
As for a chilling effect, the worse one I see is maintainer burnout from having to deal with PRs like this.
What I'm saying is that there are probably many, many authors who realise that and wouldn't risk contributing (therefore having a chilling effect). Kinda makes the project "open source-ish". All the legal infrastructure, none of the community norms.
You put the effort into a PR, and it gets bounced due to conflicts with some other codebase you don't have access to.
I think that's what they call a chilling effect.