Let’s not overthink this.
From my vantage point, it appears the intention was to release the initial subset of features (writing assistance, Genmoji, etc) with 18.0. For whatever reason, that became impossible within the time constraints dictated by the need to have a complete 18.0 in time to install on new iPhones coming off the assembly line beginning towards the end of August.
At the same time, it would have been hugely injurious to Apple’s stock price if they were to ship iOS 18 with no Apple Intelligence AND no visible roadmap to when it would appear. So they needed to get it out there now, but with an extended date for it’s release. Hence an 18.1 beta this week. This buys them 3-5 additional weeks to get things as polished as possible and keeps the new iPhone release date on schedule.
My thinking is that the 18.1 beta will be available to all users, just like betas are even they have one feature that are not available on all phones (think Standby mode).
We’ll know soon!