Alas, the Nexus Guide is still a bit unclear from some parts. The chapter called Nexus Process Flow contains this:
Nexus Sprint Planning: Appropriate representatives from each Scrum Team meet to discuss and review the refined Product Backlog. They select Product Backlog items for each team. Each Scrum Team then plans its own Sprint, interacting with other teams as appropriate.
Nexus Sprint Planning chapter tells:
During Nexus Sprint Planning, appropriate representatives from each Scrum Team validate and make adjustments to the ordering of the work as created during Refinement events. All members of the Scrum Teams should participate to minimize communication issues.
That means, everyone is present and should listen, but not everyone needs to communicate with the rest of the teams, just with their own. The purpose is coordinating the activities between the teams, instead of having a "town hall meeting".