Okay, sorry; it's true that in some contexts, like dialogue, indirect quotation, reported thought, narration, that kind of thing, it's more acceptable. And there may be others I'm not thinking of.
But I would still say that it wouldn't then be called a run-on sentence. That term, at least in my experience, is used only to denote a punctuation mistake, usually in expository writing, academic essays. In that context, the punctuation rule may actually be stricter in English than in German (for a change).
So for the benefit of German speakers writing in English, particularly in school and university settings, the definition or explanation should reflect (a) that it's considered a mistake, and (b) that it's not about the length of the sentence in absolute terms, but about whether two clauses (even short ones) are properly connected.
If Satzreihe doesn't do that, maybe someone can suggest something that does.