If community patches are licensed under MIT or BSD, all CockroachDB has to do is include those license texts in their proprietary distribution, which is the only requirement by both licenses. Proprietary derivative works are allowed under MIT and BSD.
That's not how it works. Each commit is a "derivative work" of the last. If today a codebase is licensed under MIT, tomorrow you can change a single line of code, and this produces a brand new work under US and international copyright law. You can license this work under any license, provided you comply with the MIT license of the work that has been derived.
