I'm talking about casual language-usage, like this headline, not commercial usage. Open and source are regular words, and they still are regular words when used together. Trademarks can't force people to talk as they want, it can only force company to not sell everything as they wish.
Yes, the term can't be trademarked, because they're both common English words used in combination (and that combination has a somewhat older, unrelated meaning, which further contributes to making the term untrademarkable).
But this isn't a trademark issue. No one has trademarked the word "carrots" either, but if someone were to sell pencils under the label "carrots", people would be understandably confused and annoyed.
It's the same thing here: don't call it open source if it's not. The software industry relies on that term having a specific, well-defined meaning. That meaning is widely agreed on, which, again, is why Numworks themselves is not claiming their stuff is open source.
