Are Intel's claims of their Thunderbolt cable technology what they seem ?

The claim is 40Gbps, but it seems that the figure is calculated based on two way simultaneous streams (which is fair, given the possibility to daisy chain devices), but not quite upfront.

I ask because I wanted to know why external NVMe enclosures had such dire speeds, even through Thunderbolt interfaces.

If this is the case then a Thunderbolt connected device could only theoretically accept ~2.5GBps speeds (20Gbps), which is a real bottleneck when these NVMe speeds can push over 6GBps.