The trouble is that it _isn't_ the "global idea behind it" - the idea behind open source is the unrestricted freedom to modify and reuse. Closed source, source available, and open source are quite distinct from one another. Terminology sometimes has well established meaning and that can be very important for effective communication.
I actually like proprietary source available software, but it isn't the same thing as open source and anyone claiming otherwise is simply ignorant of the very well established meaning of that term. Pedantry can be called for, particularly when a monetary incentive exists to confuse and deceive. Consider for example that the definitions of many food products are defined in law and regulated to protect the consumer from deceptive vendors.
(To their credit, Timescale gets the terminology right and I appreciate that. It's people in the HN comments section that are incorrectly throwing the term open source about and completely missing the point.)