It looks like they very carefully did not claim it to be open source :)

As somebody who typically dislikes the Elastic license, this is really a great use of it:

- it used to be completely closed source, so it's not taking any previous contributions and putting a more restrictive license on it;

- there is clearly quite a bit of thought in choosing this license over something like AGPL (complete with somebody taking to themselves on GitHub), and the license was chosen based on the goals;

- the software in question is really not that useful for non-commercial uses anyway, so people who want to freeload doesn't have that much to stand on

Even though I'm not a user (and don't see myself using it in the future), I'd nevertheless be happy for them and hope they do well :)