Rocket chat had closed federation before, so likely very little changes with this

The blog says:

building its new federation capabilities on the Matrix protocol to allow its users to communicate with users on other platforms.

The Rocket.Chat adoption of Matrix makes it simple for organizations to easily connect with external parties, whether they’re using Rocket.Chat or any other Matrix compatible platform

Which I think suggests otherwise. It seems like they’re hinting at being able to interoperate with a platform self-hosting Matrix or using one of Vector’s hosting services.