Siri is integrated into iOS, macOS, watchOS and tvOS, and developers have to make use of a library called SiriKit that allows them to exchange information with the wizard so that when it is invoked and asked for something that App can do or contain, the wizard knows where to look.
However, this is all the integration that Apple offers. The company improved the situation with the App Shortcuts, which integrated with the voice assistant allows you to do quite amazing things with a simple voice command. In order to go further, however, much more would be needed. Something, like an operating system, called SiriOS, would allow third-party Apps developers to integrate Siri into their programs much more closely than before.
The rumor comes from a study called Voice Tech 2019 from an investment company called Mangrove Capital Partners. They tell us that Siri could become SiriOS in 2020, in the presentation of what iOS 14 would be, along with other news presented in the month of June of that year.
The truth is that all this sounds quite undefined and it is not very clear that the integration of Siri in any Apple device had much to do with its own operating system. We will have to wait almost a year to find out.