No, most people define "open source" as it is defined by the Open Source Institute.

That is probably why this project was careful to define itself as "Open" and did not use the term "open source". It is licensed under Elastic v2 which is generally known as a "source available" license

I don't see any problem with this as long as it is not advertised as open source, which it is not. Of course they are entirely within their rights to license their code as they see fit. I like to use open source software but I absolutely prefer source-available software over closed source solutions like their competitors (AWS, Azure, and GCP)

This is a good reminder to check the license before you adopt something. Just because the code is on Github, doesn't mean you can do anything with it.

You're certainly entitled to define it that way or anyway you want. Just be wary a lot don't share your definition, so using a more explicit and accurate term like FOSS is probablyyy better.