'Open Source' is a meaningless term
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In short, EnterpriseDB is a company that provides a proprietary version of PostgreSQL that is a drop-in replacement for Oracle. EnterpriseDB contributes a lot of work back to the PostgreSQL project, but its own project is not open source.
'Open source' was coined as a business-friendly alternative to the term 'free software' by Eric Raymond. People conflate 'open source' and 'Free Software', but Free Software has always meant something else.
By the Free Software definition, EnterpriseDB is not a 'free software' company, because it fails to provide freedoms 1, 2, and 3. The fact that they contribute back to the project is irrelevant.
However, open source is not so clear-cut. Yet some people perist in claiming that EnterpriseDB is not an 'open source' company...seemingly on the grounds that they don't release their work as open source. I'm afraid that's the definition of Free Software, not open source. The term 'open source' was crafted to be business-friendly, and it succeeded at that. But it also muddied the waters, leading to confusions like this.