This is the most direct explanation. The blog post avoids talking about this at all, but I'm guessing that's to keep the post informative and not let it degrade into a database flame war.
As a MySQL DBA for the past 20 years it was practically the only choice until Oracle bought Sun, then it was instantly radioactive. A shame because the fears people had were unfounded and some of the best development work has been done since then.
Postgres already had cachet before Oracle bought MySql, but it was seen more as a small scale oracle replacement for people who were serious about data integrity or for lower volume OLTP work in the web sphere. The MySql acquisition coincided with performance gains by Postgres that made it more applicable to web scale OLTP workloads, and JSONB support was really the nail in the coffin.
As a MySQL DBA for the past 20 years it was practically the only choice until Oracle bought Sun, then it was instantly radioactive. A shame because the fears people had were unfounded and some of the best development work has been done since then.