Okay, not that it's the morning how about I try this again. Here are my points in list form:
1. It's not 2018 yet. With the games coming out in November they have plenty of time to change it before it becomes active. As for them not mentioning it, eh, maybe this time they didn't feel it was necessary. Or maybe they do plan on using Sun & Moon's dex for a while after USUM's release to give the competitive scene time to settle with what the new games have to offer.
2. They only mention Sun & Moon, no mention of Ultra Sun & Ultra Moon. All that info only relates to Sun & Moon.
3. To avoid spoilers they didn't want to give dex details. It's probably safe to say every Pokemon that was available in SM would be in USUM.
4. They have always expanded upon the regional dex in the third versions, why would they not now? In addition the Alolan Pokemon are generally considered a large success (and it's one of this gen's newest gimmicks) so why wouldn't they make more?
5. I'm assuming a department in the Pokemon Company made these schedules, not GF who are making the games. Infact, GF doesn't rarely has competitive first on their mind. Why is this an important distinction? There could be some communication issues which explains why Ultra Sun & Ultra Moon wasn't mentioned. Maybe GF are till deciding what new Pokemon to add (and hopefully working better on distribution) or maybe there has been some trouble with development and they might be pushing the games back and we just don't know yet.
I think we should just agree to disagree. It's certainly questionable why the official rules wouldn't make any mention to Ultra Sun & Ultra Moon, but since it doesn't we can't say it indicates anything about it either.