I don't know the exact argument happening here, but I think it's more nuanced than that. All of the Apple operating systems share the same core. They're all running the Darwin kernel. iOS and iPadOS share the same frameworks, filesystem, etc. It would be mildly crazy for Apple to maintain two distinct operating systems that share so many components.
If anything, I would say it's more like Linux distributions than operating systems. iPadOS and iOS are different interfaces on top of the same core operating system.
But who knows, I've been wrong before. Maybe they did fork iOS in 2019 and have been independently developing separate operating systems and trying to reach feature parity with each other. I don't know how Apple works.