Author here: I completely agree that it applies to any database and I tried to mention it in the article. The reason I wrote about MongoDB is:

- I wrote about it before and could compare results from my previous post

- It's popular and there are a lot of public instances of it

- MacKeeper exposed 13 million user accounts through their public MongoDB instance

And I actually wrote a follow-up post on Memcached to highlight the same issue: https://blog.shodan.io/memory-as-a-service/