> Because checks in Error Prone are self-contained and written against the javac abstract syntax tree ... it is relatively easy for developers outside the team to contribute checks... As of January 2018, 733 checks had been contributed by 162 authors.

Somewhat interestingly, Clippy is an analogous tool for Rust and has 181 contributors now, more than Error Prone. https://github.com/rust-lang-nursery/rust-clippy

Why is that relevant to this post?

Because it provides a comparison point to a statistic in the article? GP was pretty explicit about how it related to the article

Might be apples to oranges, if one tool is internal and the other isn't.