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How is that in any way surprising? The more resources you have, the more custom-tailored you can make your tools. And I can't imagine it's a shock that data workflows aren't a completely-solved-for-all-usecases problem at this point...

Don't forget Chronos.

http://mesos.github.io/chronos/

Isn't Chronos more of a distributed scheduler ("fault tolerant cron") than a workflow engine?

Distributed job scheduler is, in a larger sense, a workflow engine. It will need to understand what is the next job to trigger, handle different situation where job fails\timeouts and do the retry if specified.

which was also started at Airbnb.

drake[0] from factual falls into this category as well.

[0] - https://github.com/Factual/drake

Airflow reminded me a lot of taskflow from the OpenStack project.

Well, Facebook spun PHP off into Hack if that counts.

All a small framework called 'Ruby on Rails', released by Basecamp.

Hey, isn't a DSL your own language, strictly speaking? So any internal company-specific XML schema etc...