I would say it's almost certainly a mistake or some side-effect of their system that rolls out updates where they don't happen exactly simultaneously.
Remember that Apple is also pushing that update out to serve their iPhones that cannot get iOS 26. Even if I was to maximize my cynicism, I don't think they presently use security point releases in the manner you are describing.
I don't think we can really ascertain intent, Apple has a long history of "the feature update IS the security update".
This partly relies on the "just update bro" attitude of sufficient fanbois to achieve upgrade momentum. Otherwise, let's be honest, no one would update, ever, our phones are too personal to be changing constantly.
This "bug" has been there for 2-3 days now. If it was a bug with their software delivery system, I assume it would have been fixed by now, it's affecting many people (with plenty of message board complaints to prove it).
Yes. Following a transition period of 2-3 months after each new major OS version, they withhold security updates for older OS versions from devices that could update to the new major version. The current 18.7.3 will likely be the last iOS 18 update made available to iPhone 11 and up, while the iPhone XS and XR, which can’t update to iOS 26, will continue to receive further security updates for iOS 18. This mode of operation has been the case for many years now.
Yes. Until a few years ago after a new major iOS version was released (eg. 26 for this year) the last major version (eg. 18 for this year) stopped getting updates, at least for phones that had access to the newer version. That changed a few years ago so that there was a period of overlap where both got updated.
For instance, look at the release history for iOS 12 and 13:
Nice though cosmetics are the least of my issues with 26. Usability really tanked across the OS, crap ton of baffling choices that make it much harder and unintuitive to use.
I don't like transparency but for me it's not the main problem with iOS 26, the main problem is that new controls (and their placement) waste too much space on a small iPhone SE screen.
Remember that Apple is also pushing that update out to serve their iPhones that cannot get iOS 26. Even if I was to maximize my cynicism, I don't think they presently use security point releases in the manner you are describing.