I've seen far, far more cases of document databases being used in situations that are entirely inappropriate, than in situations where they are ideally suited.
I'd argue that the situations in which it's a good choice to use a document database are pretty rare, and that you're almost always better off using a bog-standard relational database.
I think the author of the post has made a good point, that many seem to forget - it is always a trade-off whichever tool you use.
What does "realtime" have to do with the datastore?
Whereas with SQL you have the option of using cache layers (which you could say is NoSQL anyway), message queues, naive timely polling and log monitoring.
With NoSQL the solutions are just simpler. For example Firebase has a tutorial for creating a real-time chat [1] - it takes 5 minutes.