So apple might have had USB3 speeds support in the A15 specifically for the mini, or they might have included USB3 support for a while in all SoC and enable it selectively for segmentation purposes.
That would gel with the comparison between the iPad Air 5th gen and the iPad Pro 5th gen: they both use the desktop-class M1, but the Pro supports full TB3 and USB4, while the Air only supports 10GB.
Honestly, I'd expect any Armv8.3+ or Armv9 to have USB PD/USB-C, Thunderbolt, etc. on the chip itself. USB PD is even halfway-or-more supported on some budget Cortex-M targets. Apple, though, is a special case; the chipmaker and developer are the same, so they have the freedom to say "no USB-C on mobile chips" and include only the features they want.
My post was pure speculation and a little giving them the benefit of the doubt.
So Apple is using a PCIe USB 3.0 controller for USB 3.0 iPads.
They could've easily done that on the iPhone 15, so it's market segmentation.
Adding additional chips/additional controllers is not "easily" done. The iPads have more space to spare on their boards, and have larger batteries that can absorb the additional power requirements for running those additional chips much easier.
