I think the important thing to note here is that there are lots of different ways to use any given tool that can fit your use case without being an atrocity.
And they did it because of maintenance problems not (just) performance (from the link you provided).
But they also said "Postgres was picked over Cassandra for the key-value store because Cassandra didn’t exist at the time. Plus Postgres is very fast and now natively supports KV."
Which isn't patronizing like the article OP linked.
Thanks for that... I think it depends on one's needs... it would surprise me if Wix as using MySQL without sharding data either by route and/or site.
Personally, I'd be inclined to use PostgreSQL for most things once they have an in the box solution for replication and failover that doesn't require a commercial contract for a sane solution. Every time I've used mySQL (admittedly not for about a decade), I wind up with a lot of WTF moments. If I'm paying for it, may go with MS-SQL, or a hosted "cloud" provider as tbh, no interest in being a DBA.
I really like RethinkDB's approach as a no-sql solution, and their admin interface is second to none in this space. If you really need more scale (more than around 20 servers or so), then you should probably be using Cassandra, and/or supplementing with the likes of ElasticSearch.
In any case, if you want real-time data for hundreds of millions of users, you're going to have data partitioning and redundant replication and caching solutions.
I worked at GoDaddy on Website builder while they were transitioning their publishing target to C*, and have to say it performs incredibly well as basically a distributed file system in the role described in the post article.
I think the important thing to note here is that there are lots of different ways to use any given tool that can fit your use case without being an atrocity.