It's useful to note that the term "open source" does not require an OSI-approved license. It is conventional that the term implies an OSI license, but there are projects that are undeniably open source which are not issued under an OSI license. SQLite being one of the most prominent ones.
All that can be said today formally about a project is whether or not it's OSI-licensed, rather than whether it's open source. So companies such as GitLab can claim they are "open source" legitimately without using an OSI license--as can SQLite.
GitLab can only fairly claim they are "open source" because they do have a fully-functional core of the project that is truly open source.
Whether a license is specifically OSI-approved, isn't the point. The point is that "open source" means something as a term, and that is widely respected as the standard. If someone claims a license to be "open source" when it doesn't meet the definition, they can and will be rightly criticized for being deceptive in using the term wrongly.
> there are projects that are undeniably open source which are not issued under an OSI license. SQLite being one of the most prominent ones
SQLite is public domain. The OSI recommends against using a simple public domain declaration, but at the same time, does effectively say "for practical purposes it more or less works, though we recommend using a license instead": https://opensource.org/faq#public-domain
Only because it isn't trademarked. Anyone who has the slightest shred of respect for open source community uses the term properly. They coined the term.
As for SQLite, it's compatible with OSI licenses. It has been folded into a larger work that's licensed under an OSI approved license countless times. There are some projects that might throw around the term public domain but I'd have to examine the details to see if it's ready to be used as part of an open source project.
All that can be said today formally about a project is whether or not it's OSI-licensed, rather than whether it's open source. So companies such as GitLab can claim they are "open source" legitimately without using an OSI license--as can SQLite.
Edit: clarity