Well, sure, language is ultimately defined by usage, so if enough of us agree that the word "broccoli" refers to source code being available, then it's legitimate to go around saying "broccoli" means "visible source", etc.
But the important question is, who is using "open source" to mean the definition I gave earlier? The answer is: pretty much everyone. Not just the Open Source Initiative (opensource.org) and the Free Software Foundation, but also: the Wikimedia Foundation, Creative Commons, the Electronic Frontier Foundation, all government agencies who have formally defined the term (at least as far as I've seen), also every major tech firm as far as I can tell, including Google, Microsoft, Amazon, Apple, IBM, Hewlett-Packard, Yahoo, Samsung, Huawei, ... this list could go on for a very, very long time :-). Basically, it's everyone.
Notice that that list includes Creative Commons -- that is, the organization that wrote the particular license that Numworks is using agrees that that license is not open source. That's a pretty clear sign, I think :-).
And, of course, Numworks themselves are careful not to call their stuff "open source" anywhere. You can bet that they would use that term if they felt they could. It has great marketing karma; everyone loves to advertise their stuff as "open source". And yet Numworks doesn't -- because they know a lot of hackers would come down on them like a ton of bricks if they tried to make that claim.
To a first approximation, pretty much every organization that produces or procures software use the term "open source" to mean the definition that the Open Source Initiative originally established.
You can claim that "open source" means something else, if you want, in the same way that you can claim that the word "broccoli" or "red" means something different from what most other people mean by those words. For you, your meaning will be correct, in your own private language.
But what I'm saying is, when the phrase "open source" appears in a Hacker News headline, most people will interpret it in the way I described. When the vast majority of people intepreret a word or phrase as having a certain meaning -- and especially when it's a term of art in a profession and all the specialists, lawyers, policy makers, etc in that profession agree on the meaning of the term -- then it's fair to say that that is what the term "means".