There's a difference between source available and open source. For example, actually being allowed to distribute modified versions is pretty damn important:
Restrictions
No Distribution of Modified Versions: You may not distribute modified versions of the software, whether in source or binary form.
No Forking: You may not create, maintain, or distribute a forked version of the software.
Official Distribution: Only the maintainers of the official repository are allowed to distribute the software and its modifications.
The official open-source definition expects more freedoms that just being able to see the source: the whole point of having the source isn't transparency, it's freedom. Freedom to fork and modify. Freedom to adapt the code to fix it and make it work for your use case, and share those modifications.
This doesn't let you modify the code or share your modifications at all.
People often use the OSI's Open Source Definition when using the term "open source". One of its criteria says "The license must allow modifications and derived works" which this license does not allow.
The short story is that the OSI failed to obtain a legal trademark in the US for the term "open source" (software), resulting in many opportunistic companies and individuals adopting the term popularized by the OSI (which was founded by Eric Raymond, Michael Tiemann and Bruce Perens).
There was controversy at the time due to it being a business-friendly spin on the ideological "free software", and I personally avoided using the term for many years as a result. Even without a trademark on the now generic term of Open Source, there is still value in the OSI brand and its stamp of approval on a license.
Those who want to be crystal clear, should probably always say OSI Approved Open Source License.
That's nice. If your goal is to ever talk to people about open source software, that's going to create a lot of unnecessary confusion.
On top of that, accepting this bolsters companies to use this kind of a definition specifically to take advantage of the mental model that many people have connecting "open source" with OSI.
If your goal is to ever talk to people about open source software, that’s going to create a lot of unnecessary confusion.
I guess that my definition of open source is not that uncommon, given that the terms "free software" and "libre software" exist and are rather well-established by this point.
Everyone has a different opinion on what that means, some people get really angry when you don't use their (or some other group's) explicit definition of the term "open source" that nobody actually owns. If they want it to mean something really specific, they should use a registered trade name with a defined meaning. But that usually implies some kind of capitalism at work, which most FOSS zealots are very much against.