Benson Leung (of Google, known from his in-depth cable reviews on Amazon) explained it quite well years ago:
www.reddit.com/r/UsbCHardware/comments/l4so0g/psa_thunderbolt_4_is_usb4/

This is no 'more capable' USB4 mode (a la USB3.2 Gen 2x2) that TB4 does not support. Both USB4 40Gbps and TB4 40Gbps are the same.

(You *can* have *USB4 hosts* with ports limited to 20Gbps if the manufacturer wants to cut costs, but most newer devices' USB4 will be 40Gbps capable.

Regardless, TB4 certification from Intel *requires* the 40Gbps capability; a USB4 device cannot be slower on a TB4 port.)

Saw the video review of this on YT (youtube.com/watch?v=WBg05rpydjY).

Not sure how it can be that using the EX400U with a TB4 port results in 1/2 the speed (~2,000MB/s reads) when compared to using it with a USB4 port (~3,900 GB/s reads)...

My understanding was that TB4 is a subset / certification verifying on the maximum capabilities of all USB4 features: a TB4 port is therefore supposed to be a USB4 port.

If this is not the case, it sounds like there is something very wrong here...