It was already not open source, hence the weasel language. "It will remain source available" is the second-most straightforward way to say "it already wasn't, but it's awkward to admit that given that we allowed you to misunderstand the license for five years".

Discussion from five years ago:

Relicensing CockroachDB June 4, 2019 (487 points, 282 comments) https://news.ycombinator.com/item?id=20097077

The blog post is a 404, here's the archive: https://web.archive.org/web/20190604173131/https://www.cockr...