>...What happens next? Geheheh! Who knows? There was only one more chapter... After that, it all stopped.
>timelines, starting and stopping. Until eventually, everything stops.
Is "everything stops" the last prophecy? Does the game "end" before chapter 7?
It's just a bizarre turn of phrase to refer to the end of a story. And both Sans and Gerson who have intimate knowledge of the world use it.

Maybe my interpretation is weird but it implies that Deltarune doesn't so much end as it stops abruptly.
Why wouldn't Gerson talk about 6 and 7? If it's the prophecy, does that mean CH6 and CH7 somehow exist outside of the context of the prophecy?

There's a number of other things that seem weird.
Chapter 5's secret boss is the last secret boss, why stop at 5?
And weren't chapters 3,4,5 were supposed to be "the full game"? How can it be the full game if 6 and 7 are missing?
Something tells me whatever chapter 5's ending is it is going to be a huge mindfuck