This was my impression as well. If you ignore the paper and just look at the source code - and carefully study Seph Gentle's Yjs-like RGA implementation [1] - I believe you find that it is equivalent to an RGA-style tree, but with a different rule for sorting insertions that have the same left origin. That rule is hard to describe, but with some effort one can prove that concurrent insertions commute; I'm hoping to include this in a paper someday.
[1] https://josephg.com/blog/crdts-are-the-future/