Presumably why fast-track Windows Insiders have been testing 20H1 for months already, if most of the platform guts have to go out in 20H1 for these devices to meet shipping goals.
20H1 of Windows 10 is essentially a different operating system than Windows 10X. My understanding is that the 20H1 focus is around standardizing what's running on Azure with what's running on desktops.
I think it behooves Microsoft today to brand 10X as a "different operating system", but it's not like it is an entirely independent fork. Some of the same "OneCore" improvements Azure needs on the one side are likely what 10X needs, so 20H1 can be (and probably is) for testing both. Microsoft can test two things.