Like most human categorizations, it's a simplification. There's no line in the sand, but there's a rough gradient or spectrum stretching from stories not related to HP at all, through stories with similar themes that make people think of HP (but don't explicitly make the connection in the text), through stories that use the HP names and characters and settings but change events drastically from the series (like this HP and the Wastelands of Time), through stories that change fewer things (like HPMOR), and ending with some that stick to canon as much as possible.

This isn't about what stories "deserve" to be called "true HP fanfiction". That would end up as a No True Scotsman fallacy. My original wording was poorly chosen in that it made you (plural) think I meant something like this.

Rather, what I'm saying is: the story up till now has been consistent in terms of lying in a particular location on this spectrum. It has been consistently presented and written that way. If, now, in a new chapter, we found out that prophecies don't really exist and are all faked, that would be an idea typical on a story at a very different location along that spectrum. It would conflict with the story so far. Readers would not enjoy it. It would be bad storytelling. Therefore I believe with high probability such a thing will not be revealed in HPMOR.