Leaving aside the sentence case in the title, the author's post didn't capitalise open source: they clearly mean source which is open to be read freely, and from the context this can clearly be read.
This is a highly nitpicky topic where terms have important meanings. If we toss that out, it becomes impossible to discuss it.
The GPL places no restrictions on how you can run the software. All meaningful licenses place restrictions — or, conversely, limit the permissions they grant — on how the code can be used, distributed, integrated with other projects, etc.
But I disagree that the meaning of Open Source is malleable. As others here said, if we want to make a new definition, we should make a new term. In my opinion, in this case, we have. It’s Source Available, which is basically “look, but don’t touch”. And as with other brightly colored things in nature, it’s generally best to avoid it.
> the author's post didn't capitalise open source: they clearly mean