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.