Despite the fact that you crippled it by not using jsonb columns.
http://www.postgresql.org/docs/9.4/static/datatype-json.html
You can also see that the very first link outside of the official documentation:
https://www.compose.io/articles/is-postgresql-your-next-json...
Explains the benefits of jsonb and how to use indexes.
If you're going to include a database, at the very least do 20 minutes of research on it. If that can't be given, then just don't include something.
That's like saying "I've just installed MongoDB and read the intro documentation page, I'm now going to benchmark it against a MySQL cluster, which I have years of experience with, and that I helped develop." (E.g.: they developed ArangoDB, so they should be experts in at least that, right?)
this illuminates the need for "experts" in the different components in the stack. it's intellectually dishonest to claim that a technology is not up to the task when it's not been properly treated in the first place.
Postgres' official documentation on json and jsonb is rather concise: http://www.postgresql.org/docs/9.4/static/datatype-json.html
The difference between the types is described in paragraph two. So, they either benchmarked Postgres while having no idea whatsoever what they were doing, or they were deliberately crippling the competition.
Neither option is confidence inspiring.
(And the first option seems sketchy, seeing how they then went and re-created the whole benchmark as classical RDBMS setup for the second postgres test.)
Biggest problem of (small) German tech companies IMO.
I worked for one myself and they had really good core technology, but that was it. The angelo-saxon companies always outplayed them, because they're just so much better at PR. Even their developers are better at this than most marketeers that burn money on a daily basis in Germany...