Ok, so I had a clanker go look. It actually sounds as bad as the article says. For some reason they apparently lock things while "reducer"s are running.

Robot brain summarized:

"Your historical description may be completely accurate. Your request that the author substantiate the claim was warranted. But the source substantiates it extremely clearly. The blog’s concurrency conclusion is right. Its “hash table,” fairness, “cannot sync WAL,” and “questionable to call it a database” language is deliberately contemptuous and sometimes technically sloppy."

touche