It's not you who's confused, it's that this part of the process is not described. We only have some hints, here:

> 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.