Maybe you didn't get the joke. The joke is not about MongoDB, but about MongoDB fanbois that care only about some very narrow definition of "performance".
The wider message is that DBs are way more complex beasts that is meaningful to test this way.
Obviously there are cases in which MongoDB is a great choice, but equally obviously tests like this should not be a reason for the choice.
I do understand that second degree, but I don't think the author is a fanboy...
At work, we're also Mongodb users. If tomorrow we try to benchmark against Cassandra, performance will probably be the selling point. I don't think it's absurd to compare mongodb and rethinkdb, they're very similar dbs.
As for the benchmark, I agree it doesn't explore every facet of both databases and focus on performance... That's what benchmarks do.
The intent of the author may have been to challenge his existing choice (mongodb) which is a good thing. The (corrected) results may lead to: expect no performance gain if we migrate to RethinkDB... What's wrong with that?
The wider message is that DBs are way more complex beasts that is meaningful to test this way.
Obviously there are cases in which MongoDB is a great choice, but equally obviously tests like this should not be a reason for the choice.