“Cheerios” is a made-up product name, and without knowing the definition of it, one couldn’t tell what it means. (I had to look it up, in fact.) In contrast, “open” and “source” are generic words, and in combination they can be used and understood without knowing any of the FOSS history.
That’s actually how I was introduced to the term: I always understood “open source” to just literally mean “the source code is openly accessible” – no more, no less. Even after having learned about the FOSS aspect later on, I continued to interpret “open source” in that literal sense.
Since this debate is a bit … heated, let me add that I’m not interested in making any general claims with my statement – I’m rather sharing my own perception to contribute a data point to how language can be used and understood differently, depending on where people come from.