As I understand it, they do exactly what you say. TFA is about optimizing the last chunk's loop.
> If you called rope.offset_to_point(7234), the Rope would traverse its SumTree to find the Chunk that contains offset 7234 and then, on that Chunk, it would call offset_to_point again.
And here:
> while the Rope can get us to the right Chunk in O(log(n))
I would guess that each node of the SumTree includes the number of newlines of all the chunks before it.