Jeff,
We're experiencing the same problem the original poster had in 2017 except we're running 9.3P8.
The KB did not seem to cover us as it mainly dealt with interface and IP issues. The source cluster nodes can ping the desination and vice versa. The destination is even able to continue to "pull" previously defined snapmirror updates.
However, from sysmgr on the source we cannot create new snapmirror relationships to the destination and the cluster peer status on the source for the destinaltion says "pending". On the destination for the source the cluster peer status is "available".
Again just like the original poster, we noticed this after adding 2 nodes to our previously 4 node cluster.
Thanks,
Steve