Well, to your point about doing it "much earlier", no. They still had to finalize the design and construction prototyping of the phone and set up the lines.So, the new phones are due to be released on September 20th, Apple start bundling the internal GM release onto those devices starting a month beforehand. So your cutoff date is actually the usual cutoff date, which is why Beta 8 was last beta and will be the GM.
If you're right about the tariffs, then they would have done it MUCH earlier.
The 2 features re-introduced, are not significant enough to warrant a rush release of 13.1 for the day of the phone release. a x.1 release tends to be over a month after a x.0 release, but it could be that due to rumoured new Watches, or something they will announce at the event, but not released the same day as the new phones, that they're wanting that x.1 release earlier than normal, possibly the end of September, or early October. This is much more likely than your Tariffs goes.
Also, you keep missing out the point that it makes no real sense that they would rush a x.1 beta to release in a less than 4 week period for Developers to test their apps, as some are massively broken in 13.1 b1 and already seeing them not being resolved in b2. Adobe Creative Cloud & Fitbit still seem broken and that's a big one to have broken still.
I am done with this argument for now. I think the telltales are all there for a general release of this software in 2 weeks' time. Time will tell if my position is correct or if you are.