I think Google has actually removed all third-party integrations for Assistant a while ago: https://developers.google.com/assistant/ca-sunset
In other words, they have just closed down that entire ecosystem. I don't think there's an antitrust angle here: Forcing e.g. Apple to allow third-party apps or app stores seems within reach (at least in the EU); this would be more like forcing Apple to enable apps in a world where they don't even exist (anymore).