Representing message flows for lightning protocols is hard, because we're not using turn-based protocols: peers may be sending messages at the same time. It is very useful though to find a suitable way of representing protocol flows, to be able to discuss edge cases and implement the corresponding test cases.
The following diagram perfectly represents the fact that messages can be interleaved. There is no way to misinterpret it since Bolt 8 guarantees that messages are ordered. But it is hard for the reader to match a received message with the place where it was sent.