[glennfle]

I found it in the spec in section 2.1.5. (Referring to the May 19, 2021, clean revision of USB4 1.0.)

"A USB4 host or USB4 peripheral device can optionally support interoperability with Thunderbolt 3 (TBT3) products. "A USB4 hub is required to support interoperability with Thunderbolt 3 products on all of its DFP . A USB4-Based Dock is required to support interoperability with Thunderbolt 3 products on its UFP in addition to all of its DFP. "When interoperating with a TBT3 product, Thunderbolt Alt Mode is established on the link between products. The USB Type-C Specification describes how a USB4 product negotiates and enters Thunderbolt Alt Mode."

So, accurate!

However, my question is about major brands. Will Lenovo, Dell, etc., go for USB 4-only instead of USB 4/TB 4? I guess we'll find out.

But my understanding would be that a Thunderbolt 4 controller connected by a USB 4 cable to a USB 4-only device running USB 4 for 20 Gbps would still work through Thunderbolt 4’s backwards compatibility across all USB modes.