But I'm not totally clear on why a switch from Postgres to MySQL was warranted.
Just because Postgres can do stored procedures well doesn't mean that it's effectively a stored procedure server. It also happens to handle SQL pretty well too. :)
Given their existing investment in Postgresql, I can't imagine they suddenly had more experience internally with MySQL, though the CTO may have been the only person with any horizontal DB scaling experience since Etsy didn't scale before.
I've seen the pattern of "CTO changes tech to something he/she is familiar with" many times. Sometimes that's good; mostly, it annoys the teams who are now less efficient. We certainly don't have enough info here to know. The scaling argument (his knowledge thereof, not Postgresql's "inability") is pretty compelling.
Also, knowing how to scale DB X and not knowing how to scale DB Y often means scaling DB Y is a science project.
I feel bad for baiting people into psychoanalyzing Etsy's team.
You can apparently do that with Galera, but I haven't seen that in production.
What they're referring to is most likely MySQL's half-baked "circular replication". Some people claim it works for them. Personally I tried it once and the usual MySQL replication issues (desync, InnoDB deadlock) would create much more uncomfortable situations than in a Master/Slave setup.
With master/slave you at least stand a chance of having one half of the pair survive through a problem. With circular replication the cluster would lock up hard almost every time for us, and - to add insult to injury - leave the pair inconsistent after recovery.
PostgreSQL has many, many strengths over MySQL, but historically replication wasn't one of them. Built-in replication didn't come to PostgreSQL until version 9, and until then various poorly scalable trigger based log shipping schemes (like Slony) were used. I wouldn't be surprised if PostgreSQL's built-in replication system with version >= 9 works quite well nowadays, but I think at the time Etsy were considering the switch there was still quite a lot of FUD around replication.