On iPads, the "base" iPad is on USB2 speeds, the mini supports 5Gbps, the air 10, and only the pro supports 40Gb USB4. And while SoC differences could explain differences between the iPad (A14) and Mini (A15), the 5th gen Air uses the same SoC as the 5th gen Pro which already did support 40Gb USB4. So it's not an SoC limitation.
Although they ended up putting the 48MP main camera on the non-pro and I thought that'd remain pro-exclusive as well so...