It's not so much that Elastic is saying it as a lot of people doing the supposed wrong the advert-article describes.

I've seen some examples of people using ES as a database, which I'd advise against for pretty much the reasons TFA brings up, unless I can get by on just a YAGNI reasoning.

It will also depend a lot on the type of data: Logs are an easy yes. Something that required multi-document transactions (unless you're able to structure it differently) is a harder tradeoff. Though loss of ACKed documents shouldn't really be a thing any more.