I think this article is targeted at the latter: pre-MVP and just post-MVP. For those startups, having two databases with one dedicated to a backend analytics system reeks of premature optimization.
If you grow to be more than 2-people, then taking a mirror dump of the prod db to run queries against is pretty trivial effort.
Set up a slave of your MySQL or Postgres database (no the slave is not a backup, but dumping it regularly is an easy first step to a very basic backup setup), hosted in a different data centre, and if you're a two person company you now also have somewhere to run analytics whenever you feel ready.
It can be <1 hour effort and a few tens of dollars a month in extra costs for a small system, yet makes a tremendous difference in resilience and gives you that db to run analytics against "for free" whenever you do need it.
Its free software, you don't have to pay for two instances of Oracle.
One thing that will quickly kill a biz is combining the functions of PROD and DEV/TEST. Making the DEV/TEST box the DEV/TEST/REPORTS box is not a big deal, and you can't run a (real) biz without a DEV/TEST box.
Eventually, nothing can match the performance of storing binary blobs on a cluster. But that only becomes worthwhile if you database is significantly larger than a terabyte. And I'm only talking about the operational "core" database, not your "data warehouse" (the log dumping ground, which should be split off when your database gets to be a few dozen gigs).
Meanwhile, mysql has big advantages :
1) can do basic optimization with "ALTER TABLE", even (mostly) live.
2) you can mix PROD and DEV/TEST (though obviously you need to use good judgement). Obviously you should also have a DEV/TEST instance for actual testing. Sometimes you want to run a test quickly against PROD though. Adding a slave, having it sync and then running against the slave is a joy.
3) creating reports is quick, customizable and everything you want.
4) It's "idiot-friendly". Employees can ramp up to the structure in a mysql db in 2 weeks flat. Try that with custom document stores.
5) It's typesafe and relational safe (if correctly designed), with the advantages that brings : significantly less weirdness in the database.
6) Phpmyadmin. Mysql workbench. Django. Php ...
I'm even going to argue that the GP's argument, that running analytics on PROD can get you fired, is not just wrong, it's actually an advantage of using mysql. (And the open source SAP database can run "live" analytics. You just can't believe how great that is for dashboards)