Ben Lovejoy has the slightly confusing report.
It seems M1 macs don’t support 10gb USB 3.1 Gen 2 and only 10gb for USB 3.2 which should reach 20gb, so don’t officially support usb 4 which would have to include both of those.
So it basically supports the 5gb of Usb 3.1 Gen 1, and 3.2 is dual, so 2 5gb, but full thunderbolt 4 40gb.
Intel macs supported USB 3.1 Gen 2 at 10GB, but also only supported USB 3.2 at 10gb.
I don’t know if this is licensing, saving money or just bad implementation on their own chip design. This is a huge mistake and one I wish Apple would deal with in the future (i bet it is hardware and not software).