Well, the poster explicitly talks about 2003 here: „ In 2003, MySQL was much more widely used than PostgreSQL. MySQL was also potentially faster as it did not implement all features of the SQL standard“
Didn't very early mysql play fast and loose with the concept of actually syncing to disk? That was also fast. Web scale fast. :)
At the time, MySQL was also the default for every PHP backed webhost provider. That's the market they lost. They're the Perl of DBs.