One thing to note in the Language Features section you are referring to, which I agree is more impressive than typical, is that it includes two medium-sized changes (immutable arrays and labeled tuples) that were designed and developed by Jane Street and later proposed for upstreaming (for immutable arrays, @otini from Tarides did the upstreaming).
This might be the first time this happens since Jane Street switched from upstream-first to fork-first development, and it required efforts on both sides, both the technical work of writing and reviewing the upstream change proposals (design and implementation) but also finding processes to do this that work for everyone involved. It’s more difficult than it looks like, and I’m happy with this particular pair of results.