It looks like the paid edition of Zulip is the same as the open source version so it's not open core. Of course if Zulip becomes too popular then we may see AWS GroupChat followed by a license change...
AWS already has Chime. They don’t need anything else.
Zulip supports itself by selling hosting (at zulipchat.com) and support contracts for on-prem installations. Everything is a part of the open source project; no proprietary add-ons.